(Meth)acrylic esters of polyalkoxylated trimethylolpropane

ABSTRACT

The present invention relates to novel (meth)acrylic esters of polyalkoxylated trimethylolpropane of the formula 
                         
where EO is O—CH2-CH2-
     PO is independently at each instance O—CH2-CH(CH3)- or O—CH(CH3)-CH2-   n1, n2 and n3 are independently 4, 5 or 6,   n1+n2+n3 is 14, 15 or 16,   m1, m2 and m3 are independently 1, 2 or 3,   m1+m2+m3 is 4, 5 or 6,   R1, R2 and R3 are independently H or CH3,   a simplified process for preparing these esters and the use of reaction mixtures thus obtainable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the U.S. national phase application of International ApplicationNo. PCT/EP03/06054, filed Jun. 10, 2003, which claims the benefit ofGerman patent application No. 102 25 943.7, filed Jun. 11, 2002, andGerman patent application No. 103 15 336.5, filed Apr. 3, 2003.

The present invention relates to novel (meth)acrylic esters ofpolyalkoxylated trimethylolpropane, a simplified process for preparingthese esters and the use of reaction mixtures thus obtainable.

Swellable hydrogel-forming addition polymers, known as superabsorbentpolymers or SAPs, are known from the prior art. They are networks offlexible hydrophilic addition polymers, which can be both ionic andnonionic in nature. They are capable of absorbing and binding aqueousfluids by forming a hydrogel and therefore are preferentially used formanufacturing tampons, diapers, sanitary napkins, incontinence articles,training pants for children, insoles and other hygiene articles for theabsorption of body fluids. Superabsorbents are also used in other fieldsof technology where fluids, especially water or aqueous solutions, areabsorbed. These fields include for example storage, packaging,transportation (packaging material for water-sensitive articles, forexample flower transportation, shock protection); food sector(transportation of fish, fresh meat; absorption of water, blood in freshfish/meat packs); medicine (wound plasters, water-absorbent material forburn dressings or for other weeping wounds), cosmetics (carrier materialfor pharmaceuticals and medicaments, rheumatic plasters, ultrasound gel,cooling gel, cosmetic thickeners, sunscreen); thickeners for oil/wateror water/oil emulsions; textiles (gloves, sportswear, moistureregulation in textiles, shoe inserts); chemical process industryapplications (catalyst for organic reactions, immobilization of largefunctional molecules (enzymes), adhesive for agglomerations, heatstorage media, filtration aids, hydrophilic component in polymerlaminates, dispersants, liquefiers); building and construction,installation (powder injection molding, clay-based renders,vibration-inhibiting medium, assistants in relation to tunneling inwater-rich ground, cable sheathing); water treatment, waste treatment,water removal (deicers, reusable sandbags); cleaning; agricultureindustry (irrigation, retention of meltwater and dew precipitates,composting additive, protection of forests against fungal and insectinfestation, delayed release of active ingredients to plants); fireprotection (flying sparks)(covering houses or house walls with SAP gel,since water has a very high heat capacity, ignition can be prevented;spraying of SAP gel in the case of fires such as for example forestfires); coextrusion agent in thermoplastic polymers (hydrophilicizationof multilayer films); production of films and thermoplastic moldingscapable of absorbing water (for example agricultural films capable ofstoring rain and dew water); SAP-containing films for keeping freshfruit and vegetables which can be packed in moist films; the SAP storeswater released by the fruit and vegetables without forming condensationdroplets and partly reemits the water to the fruit and vegetables, sothat neither fouling nor wilting occurs; SAP-polystyrene coextrudatesfor example for food packs such as meat, fish, poultry, fruit andvegetables; carrier substance in active-ingredient formulations (drugs,crop protection). Within hygiene articles, superabsorbents are generallypositioned in an absorbent core which comprises other materials,including fibers (cellulose fibers), which act as a kind of liquidbuffer to intermediately store the spontaneously applied liquid insultsand are intended to ensure efficient channelization of the body fluidsin the absorbent core toward the superabsorbent.

The current trend in diaper design is toward ever thinner constructionshaving a reduced cellulose fiber content and an increased hydrogelcontent. The trend toward ever thinner diaper constructions hassubstantially changed the performance profile required of the waterswellable hydrophilic polymers over the years. Whereas at the start ofthe development of highly absorbent hydrogels it was initially solelythe very high swellability on which interest focused, it wassubsequently determined that the ability of the superabsorbent totransmit and distribute fluid is also of decisive importance. It hasbeen determined that conventional superabsorbents greatly swell at thesurface on wetting with liquid, so that transportation of liquid intothe particle interior is substantially compromised or completelyprevented. This trait of superabsorbents is known as gel blocking. Thegreater amount of polymer per unit area in the hygiene article must notcause the swollen polymer to form a barrier layer to subsequent fluid. Aproduct having good transportation properties will ensure optimalutilization of the entire hygiene article. This prevents the phenomenonof gel blocking, which in the extreme case will cause the hygienearticle to leak. Fluid transmission and distribution is thus of decisiveimportance with regard to the initial absorption of body fluids.

Good transportation properties are possessed for example by hydrogelshaving high gel strength in the swollen state. Gels lacking in strengthare deformable under an applied pressure, for example pressure due tothe bodyweight of the wearer of the hygiene article, and clog the poresin the SAP/cellulose fiber absorbent and so prevent continued absorptionof fluid. Enhanced gel strength is generally obtained through a higherdegree of crosslinking, although this reduces retention performance ofthe product. An elegant way to enhance gel strength is surfacepostcrosslinking. In this process, dried superabsorbents having anaverage crosslink density are subjected to an additional crosslinkingstep. Surface postcrosslinking increases the crosslink density in thesheath of the superabsorbent particle, whereby the absorbency under loadis raised to a higher level. Whereas the absorption capacity decreasesin the superabsorbent particle sheath, the core has an improvedabsorption capacity (compared to the sheath) owing to the presence ofmobile polymer chains, so that sheath construction ensures improvedfluid transmission without occurrence of the gel blocking effect. It isperfectly desirable for the total capacity of the superabsorbent to beoccupied not spontaneously but with time delay. Since the hygienearticle is generally repeatedly insulted with urine, the absorptioncapacity of the superabsorbent should sensibly not be exhausted afterthe first disposition.

Highly swellable hydrophilic hydrogels are especially polymers of(co)polymerized hydrophilic monomers, graft (co)polymers of one or morehydrophilic monomers on a suitable grafting base, crosslinked celluloseor starch ethers, crosslinked carboxymethylcellulose, partiallycrosslinked polyalkylene oxide or natural products which swell inaqueous fluids, for example guar derivatives. Such hydrogels are used asproducts which absorb aqueous solutions to produce diapers, tampons,sanitary napkins and other hygiene articles, but also as water-retainingagents in market gardening.

To improve the performance properties, for example Rewet in the diaperand AUL, highly swellable hydrophilic hydrogels are generally surface orgel postcrosslinked. This postcrosslinking is known per se to oneskilled in the art and is preferably effected in aqueous gel phase or assurface postcrosslinking of the ground and classified polymer particles.

EP 238050 discloses (as possible internal crosslinkers forsuperabsorbents) doubly or triply acrylates or methacrylated additionproducts of ethylene oxide and/or propylene oxide withtrimethylolpropane.

Sartomer (Exton, Pa., USA), for example, sells under the indicated tradenames trimethylolpropane triacrylate (SR 351), triply monoethoxylatedtrimethylolpropane triacrylate (SR 454), triply diethoxylatedtrimethylolpropane triacrylate (SR 499), triply triethoxylatedtrimethylolpropane triacrylate (SR 502), triply pentaethoxylatedtrimethylolpropane triacrylate (SR 9035) and altogether 20 molethoxylated trimethylolpropane triacrylate (SR 415). Propoxylatedtrimethylolpropane triacrylates are obtainable under the trade names SR492 (three times 1 PO per TMP) and CD 501 (three times 2 PO per TMP).

WO 93/21237 discloses (meth)acrylates of alkoxylated polyhydric C₂-C₁₀hydrocarbons that are useful as crosslinkers. The trimethylolpropanecrosslinkers used correspond to SR 351, SR 454, SR 502, SR 9035 and SR415. These crosslinkers have 0, 3, 9, 15 or 20 EO units per TMP. WO93/21237 says it is advantageous to have 3 times 2-7 EO units per TMP,and especially 3 times 4-6 EO units per TMP.

The disadvantage with these compounds is that costly and inconvenientpurifying operations are needed for at least partial removal of startingmaterials and by-products; the crosslinkers used in the reference citedhave an acrylic acid content of less than 0.1% by weight.

Ethoxylated trimethylolpropane tri(meth)acrylates are again and againmentioned as internal crosslinkers in the patent literature, althoughonly the TMP derivatives commercially available from Sartomer are used,for example trimethylolpropane triethoxylate triacrylate in WO 98/47951,Sartomer #9035 as highly ethoxylated trimethylolpropane triacrylate(HeTMPTA) in WO 01/41818 and SR 9035 and SR-492 in WO 01/56625.

The production of such higher (meth)acrylic esters by acid-catalyzedesterification of (meth)acrylic acid with the corresponding alcohols inthe presence of an inhibitor/inhibitor system and in the presence orabsence of a solvent such as benzene, toluene or cyclohexane is commonknowledge.

Since the formation of the ester from (meth)acrylic acid and alcohol isknown to be based on an equilibrium reaction, it is customary to use onestarting material in excess and/or to remove the esterification waterformed and/or the target ester from the equilibrium in order thatcommercial conversions may be obtained.

Therefore, in the production of higher (meth)acrylic esters, it iscustomary to remove the water of reaction and to use an excess of(meth)acrylic acid.

U.S. Pat. No. 4,187,383 describes an esterification process of(meth)acrylic acid with organic polyols at a reaction temperature offrom 20 to 80° C. using an equivalent excess of from 2:1 to 3:1.

The disadvantage of this process is that the low reaction temperaturemeans that the reaction times are up to 35 hours and that excess acid inthe reaction mixture is removed by neutralization followed by phaseseparation.

WO 2001/14438 (Derwent Abstract No. 2001-191644/19) and WO 2001/10920(Chemical Abstracts 134:163502) describe processes for esterifying(meth)acrylic acid with polyalkylene glycol monoalkyl ethers in a ratioof 3:1-50:1 in the presence of acids and polymerization inhibitors and,after deactivation of the acidic catalyst, copolymerization of theresidue of (meth)acrylic ester and (meth)acrylic acid at pH 1.5-3.5, andalso the use of said residue as a cement additive.

The disadvantage with these processes is that they are restricted topolyalkylene glycol monoalkyl ethers, that the catalyst has to bedeactivated and that such copolymers cannot be used as crosslinkers forhydrogels since they only have one functionality.

It is an object of the present invention to provide further compoundswhich can be used as free-radical crosslinkers for polymers andespecially for superabsorbents and to simplify the process for preparingsubstances which are useful as free-radical crosslinkers forsuperabsorbents.

We have found that this object is achieved by an ester F of the formulaI

where EO is O—CH2-CH2

-   PO is independently at each instance O—CH2-CH(CH3)- or    O—CH(CH3)-CH2--   n1, n2 and n3 are independently 4, 5 or 6,-   n1+n2+n3 is 14, 15 or 16,-   m1, m2 and m3 are independently 1, 2 or 3,-   m1+m2+m3 is 4, 5 or 6,-   R1, R2 and R3 are independently H or CH3.

The EO or PO units have been incorporated in such a way that polyethersare formed and not peroxides.

Preference is given to esters F having the above meanings whereinn1+n2+n3 is 15.

Particular preference is given to esters F having the above meaningswherein n1=n2=n3=5.

Preference is also given to esters F having the above meanings whereinm1+m2+m3 is 5.

Particular preference is also given to esters F having the abovemeanings wherein m1=m2=2 and m3=1.

Very particular preference is given to esters F wherein R1, R2 and R3are identical, especially when R1, R2 and R3 are each H.

We have found that the object is further achieved by a process forpreparing an ester F of alkoxylated trimethylolpropane with(meth)acrylic acid, comprising the steps of

-   a) reacting alkoxylated trimethylolpropane with (meth)acrylic acid    in the presence of at least one esterification catalyst C and of at    least one polymerization inhibitor D and optionally also of a    water-azeotroping solvent E to form an ester F,-   b) optionally removing from the reaction mixture some or all of the    water formed in a), during and/or after a),-   f) optionally neutralizing the reaction mixture,-   h) when a solvent E was used, optionally removing this solvent by    distillation, and/or-   i) stripping with a gas which is inert under the reaction    conditions.

In a preferred embodiment

-   -   the molar excess of (meth)acrylic acid to alkoxylated        trimethylolpropane is at least 3.15:1 and    -   the optionally neutralized (meth)acrylic acid present in the        reaction mixture after the last step substantially remains in        the reaction mixture.

(Meth)acrylic acid in the context of the present invention comprehendsmethacrylic acid, acrylic acid or mixtures of methacrylic acid andacrylic acid. Acrylic acid is preferred.

When the ester F is desired in pure form, it can be purified by knownseparation processes.

The molar excess of (meth)acrylic acid to alkoxylated trimethylolpropaneis at least 3.15:1, preferably at least 3.3:1, more preferably at least3.75:1, even more preferably at least 4.5:1 and especially at least7.5:1.

In a preferred embodiment, (meth)acrylic acid is used in an excess offor example greater than 15:1, preferably greater than 30:1, morepreferably greater than 60:1, even more preferably greater than 150:1,especially greater than 225:1 and specifically greater than 300:1.

The esterification products thus obtainable can be used as radicalcrosslinkers in hydrogels substantially without further purification,specifically without substantial removal of the excess of (meth)acrylicacid and of the esterification catalyst C.

Unless otherwise mentioned, crosslinking as used herein is to beunderstood as meaning radical crosslinking (gel crosslinking; internalcrosslinking; cross-linking together of linear or lightly crosslinkedpolymer). This crosslinking can take place via free-radical or cationicpolymerization mechanisms or other mechanisms, for example Michaeladdition, esterification or transesterification mechanisms, but ispreferably effected by free-radical polymerization.

Hydrogel-forming polymers capable of absorbing aqueous fluids preferablyare capable of absorbing at least their own weight, preferably 10 timestheir own weight of distilled water and they are preferably capable ofachieving this absorption even under a pressure of 0.7 psi.

Alkoxylated trimethylolpropane useful for the purposes of the presentinvention have a structure as in the formula II

where EO, PO, n1, n2, n3, m1, m2 and m3 are each as defined for theesters.

The reaction of trimethylolpropane with an alkylene oxide is well-knownto one skilled in the art. Possible ways of conducting the reaction maybe found in Houben-Weyl, Methoden der Organischen Chemie, 4th edition,1979, Thieme Verlag Stuttgart, editor Heinz Kropf, volume 6/1a, part 1,pages 373 to 385.

An example of a way to prepare compounds of the formula II is to reactthe trimethylolpropane first with EO and then with PO.

This an be accomplished for example by placing about 77 g oftrimethylolpropane with 0.5 g of KOH 45% in water as an initial chargein an autoclave and dewatering the initial charge at 80° C. and reducedpressure (about 20 mbar). The appropriate amount of propylene oxide isthen added at 120 to 130° C. and allowed to react at this temperatureunder elevated pressure. The reaction has ended when no further changein pressure is observed. The reaction mixture is then stirred for afurther 30 min at 120° C. The appropriate amount of ethylene oxide issubsequently added at 145 to 155° C. at elevated pressure over aprolonged period and likewise allowed to react. After purging with inertgas and cooling down to 60° C., the catalyst is separated off byaddition of sodium pyrophosphate and subsequent filtration.

The viscosity of the polyalcohols which can be used according to thepresent invention is not subject to any particular requirements bar thatthey should be readily pumpable to about 80° C., preferably they shouldhave a viscosity below 1000 mPas, preferably below 800 mPas and mostpreferably below 500 mPas.

Useful esterification catalysts C for the present invention are sulfuricacid, aryl or alkyl sulfonic acids or mixtures thereof. Examples of arylsulfonic acids are benzenesulfonic acid, para-toluenesulfonic acid anddodecylbenzenesulfonic acid, and examples of alkyl sulfonic acids aremethanesulfonic acid, ethanesulfonic acid and trifluoromethanesulfonicacid. Similarly, strongly acidic ion exchangers or zeolites are usefulas esterification catalysts. Preference is given to sulfuric acid andion exchangers.

Useful polymerization inhibitors D for the present invention include forexample phenols such as alkylphenols, for example, o-, m- or p-cresol(methylphenol), 2-tert-butyl-4-methylphenol,6-tert-butyl-2,4-dimethylphenol, 2,6-di-tert-butyl-4-methylphenol,2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol,2-methyl-4-tert-butylphenol, 4-tert-butyl-2,6-dimethylphenol, or2,2′-methylenebis(6-tert-butyl-4-methylphenol), 4,4′-oxydiphenol,3,4-(methylenedioxy)phenol (sesamol), 3,4-dimethylphenol, hydroquinone,pyrocatechol (1,2-dihydroxybenzene),2-(1′-methylcyclohex-1′-yl)-4,6-dimethylphenol, 2- or4-(1′-phenyleth-1′-yl)phenol, 2-tert-butyl-6-methylphenol,2,4,6-tris-tert-butylphenol, 2,6-di-tert-butylphenol,2,4-di-tert-butylphenol, 4-tert-butylphenol, nonylphenol [11066-49-2],octylphenol [140-66-9], 2,6-dimethylphenol, bisphenol A, bisphenol F,bisphenol B, bisphenol C, bisphenol S, 3,3′,5,5′-tetrabromo-bisphenol A,2,6-di-tert-butyl-p-cresol, Koresin® from BASF AG, methyl3,5-di-tert-butyl-4-hydroxybenzoate, 4-tert-butylpyrocatechol,2-hydroxybenzyl alcohol, 2-methoxy-4-methylphenol,2,3,6-trimethylphenol, 2,4,5-trimethylphenol, 2,4,6-trimethylphenol,2-isopropylphenol, 4-isopropylphenol, 6-isopropyl-m-cresol, n-octadecylβ-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate,1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl) isocyanurate,1,3,5-tris(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxyethylisocyanurate, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)isocyanurate or pentaerythritoltetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate],2,6-di-tert-butyl-4-dimethyl-aminomethylphenol,6-sec-butyl-2,4-dinitrophenol, Irganox® 565, 1141, 1192, 1222 and 1425from Ciba Spezialitatenchemie, octadecyl3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate, hexadecyl3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate, octyl3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate,3-thia-1,5-pentanediol bis[(3′,5′-di-tert-4′-hydroxyphenyl)propionate],4,8-dioxa-1,11-undecanediolbis[(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate],4,8-dioxa-1,11-undecanediolbis[(3′-tert-butyl-4′-hydroxy-5′-methylphenyl)propionate],1,9-nonanediol bis[(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate],1,7-heptanediaminebis[3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionamide],1,1-methanediaminebis[3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionamide],3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionic acid hydrazide,3-(3′,5′-di-methyl-4′-hydroxyphenyl)propionic acid hydrazide,bis(3-tert-butyl-5-ethyl-2-hydroxyphen-1-yl)methane,bis(3,5-di-tert-butyl-4-hydroxyphen-1-yl)methane,bis[3-(1′-methylcyclohex-1′-yl)-5-methyl-2-hydroxyphen-1-yl]methane,bis(3-tert-butyl-2-hydroxy-5-methylphen-1-yl)methane,1,1-bis(5-tert-butyl-4-hydroxy-2-methylphen-1-yl)ethane,bis(5-tert-butyl-4-hydroxy-2-methylphen-1-yl) sulfide,bis(3-tert-butyl-2-hydroxy-5-methylphen-1-yl) sulfide,1,1-bis(3,4-dimethyl-2-hydroxyphen-1-yl)-2-methylpropane,1,1-bis(5-tert-butyl-3-methyl-2-hydroxyphen-1-yl)butane,1,3,5-tris-[1′-(3″,5″-di-tert-butyl-4″-hydroxyphen-1″-yl)meth-1′-yl]-2,4,6-trimethylbenzene,1,1,4-tris(5′-tert-butyl-4′-hydroxy-2′-methylphen-1′-yl)butane,aminophenols, for example para-aminophenol, nitrosophenols, for examplepara-nitrosophenol, p-nitroso-o-cresol, alkoxyphenols, for example2-methoxyphenol (guajacol, pyrocatechol monomethyl ether),2-ethoxyphenol, 2-isopropoxyphenol, 4-methoxyphenol (hydroquinonemonomethyl ether), mono- or di-tert-butyl-4-methoxyphenol,3,5-di-tert-butyl-4-hydroxyanisole, 3-hydroxy-4-methoxybenzyl alcohol,2,5-dimethoxy-4-hydroxybenzyl alcohol (syringa alcohol),4-hydroxy-3-methoxybenzaldehyde (vanillin),4-hydroxy-3-ethoxybenzaldehyde (ethylvanillin),3-hydroxy-4-methoxybenzaldehyde (isovanillin),1-(4-hydroxy-3-methoxyphenyl)ethanone (acetovanillone), eugenol,dihydroeugenol, isoeugenol, tocopherols, for example α-, β-, γ-, δ- andε-tocopherol, tocol, α-tocopherolhydroquinone, and also2,3-dihydro-2,2-dimethyl-7-hydroxybenzofuran(2,2-dimethyl-7-hydroxycoumaran), quinones and hydroquinones such ashydroquinone or hydroquinone monomethyl ether,2,5-di-tert-butylhydroquinone, 2-methyl-p-hydroquinone,2,3-dimethylhydroquinone, trimethylhydroquinone, 4-methylpyrocatechol,tert-butylhydroquinone, 3-methylpyrocatechol, benzoquinone,2-methyl-p-hydroquinone, 2,3-dimethylhydroquinone,trimethylhydroquinone, 3-methylpyrocatechol, 4-methylpyrocatechol,tert-butylhydroquinone, 4-ethoxyphenol, 4-butoxyphenol, hydroquinonemonobenzyl ether, p-phenoxyphenol, 2-methylhydroquinone,2,5-di-tert-butylhydroquinone, tetramethyl-p-benzoquinone, diethyl1,4-cyclohexanedion-2,5-dicarboxylate, phenyl-p-benzoquinone,2,5-dimethyl-3-benzyl-p-benzoquinone,2-isopropyl-5-methyl-p-benzoquinone (thymoquinone),2,6-diisopropyl-p-benzoquinone, 2,5-dimethyl-3-hydroxy-p-benzoquinone,2,5-dihydroxy-p-benzoquinone, embelin, tetrahydroxy-p-benzoquinone,2,5-dimethoxy-1,4-benzoquinone, 2-amino-5-methyl-p-benzoquinone,2,5-bisphenylamino-1,4-benzoquinone, 5,8-dihydroxy-1,4-naphthoquinone,2-anilino-1,4-naphthoquinone, anthraquinone, N,N-dimethylindoaniline,N,N-diphenyl-p-benzoquinonediimine, 1,4-benzoquinone dioxime,coerulignone, 3,3′-di-tert-butyl-5,5′-dimethyldiphenoquinone, p-rosolicacid (aurine), 2,6-di-tert-butyl-4-benzylidenebenzoquinone,2,5-di-tert-amylhydroquinone, nitroxide free radicals such as4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy free radical,4-oxo-2,2,6,6-tetramethylpiperidinyloxy free radical,4-acetoxy-2,2,6,6-tetramethylpiperidinyloxy free radical,2,2,6,6-tetramethylpiperidinyloxy free radical,4,4′,4″-tris(2,2,6,6-tetramethylpiperidinyloxy) phosphite,3-oxo-2,2,5,5-tetramethylpyrrolidinyloxy free radical,1-oxyl-2,2,6,6-tetramethyl-4-methoxypiperidine,1-oxyl-2,2,6,6-tetramethyl-4-trimethylsilyloxypiperidine,1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 2-ethylhexanoate,1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl stearate,1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl benzoate,1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl (4-tert-butyl)benzoate,bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) succinate,bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) adipate,bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl) 1,10-decanedioate,bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl) n-butylmalonate,bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl) phthalate,bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl) isophthalate,bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl) terephthalate,bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl) hexahydro-terephthalate,N,N′-bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)adipamide,N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)caprolactam,N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)dodecylsuccinimide,2,4,6-tris[N-butyl-N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl]triazine,N,N′-bis(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)-N,N′-bisformyl-1,6-diaminohexane,4,4′-ethylenebis(1-oxyl-2,2,6,6-tetramethyl-3-piperazinone), aromaticamines such as phenylenediamines, N,N-diphenylamine,N-nitrosodiphenylamine, nitrosodiethylaniline,N,N′-dialkyl-para-phenylenediamine, wherein the alkyl radicals can bethe same or different and may each independently contain from 1 to 4carbon atoms and be straight-chain or branched, for exampleN,N′-di-iso-butyl-p-phenylenediamine,N,N′-di-iso-propyl-p-phenylenediamine, Irganox 5057 from CibaSpezialitätenchemie, N,N′-di-iso-butyl-p-phenylenediamine, N,N′-di-isopropyl-p-phenylenediamine, p-phenylenediamine,N-phenyl-p-phenylenediamine, N,N′-diphenyl-p-phenylenediamine,N-isopropyl-N-phenyl-p-phenylenediamine,N,N′-di-sec-butyl-p-phenylenediamine (Kerobit® BPD from BASF AG),N-phenyl-N′-isopropyl-p-phenylenediamine (Vulkanox® 4010 from Bayer AG),N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine,N-phenyl-2-naphthylamine, iminodibenzyl, N,N′-diphenylbenzidine,N-phenyltetraaniline, acridone, 3-hydroxydiphenylamine,4-hydroxydiphenylamine, hydroxylamines such as N,N-diethylhydroxylamine,urea derivatives such as urea or thiourea, phosphorus compounds, such astriphenylphosphine, triphenyl phosphite, hypophosphorous acid ortriethyl phosphite, sulfur compounds such as diphenyl sulfide,phenothiazine or metal salts, for example copper chloride, copperdithiocarbamate, copper sulfate, copper salicylate, copper acetate,manganese chloride, manganese dithiocarbamate, manganese sulfate,manganese salicylate, manganese acetate, cerium chloride, ceriumdithiocarbamate, cerium sulfate, cerium salicylate, cerium acetate,nickel chloride, nickel dithiocarbamate, nickel sulfate, nickelsalicylate, nickel acetate, chromium chloride, chromium dithiocarbamate,chromium sulfate, chromium salicylate, chromium acetate or mixturesthereof. Preference is given to the phenols and quinones mentioned,particular preference is given to hydroquinone, hydroquinone monomethylether, 2-tert-butyl-4-methylphenol, 6-tert-butyl-2,4-dimethylphenol,2,6-di-tert-butyl-4-methylphenol, 2,4-di-tert-butylphenol, triphenylphosphite, hypophosphorous acid, CuCl₂ and guajacol, and very particularpreference is given to hydroquinone and hydroquinone monomethyl ether.

Particular preference is given to hydroquinone monomethyl ether,hydroquinone and alkylphenols, optionally in combination with triphenylphosphite and/or hypophosphorous acid.

Very particular preference is given to α-tocopherol (vitamin E),β-tocopherol, γ-tocopherol or δ-tocopherol, optionally in combinationwith triphenyl phosphite and/or hypophosphorous acid.

Stabilization may be further supported by the presence of anoxygen-containing gas, preferably air or a mixture of air and nitrogen(lean air).

Among the recited stabilizers, preference is given to those which areaerobic, ie those which require the presence of oxygen to fully developtheir inhibiting effect.

Useful solvents E for the present invention are particularly solventswhich are suitable for azeotropic removal of the water of reaction, ifdesired, in particular aliphatic, cycloaliphatic and aromatichydrocarbons or mixtures thereof.

Preference is given to n-pentane, n-hexane, n-heptane, cyclohexane,methylcyclohexane, benzene, toluene or xylene. Particular preference isgiven to cyclohexane, methylcyclohexane and toluene.

The esterification may be carried out by conventional preparation and/orworkup processes for polyhydric alcohols, for example the processesmentioned at the beginning or the processes described in DE-A 199 41136, DE-A 38 43 843, DE-A 38 43 854, DE-A 199 37 911, DE-A 199 29 258,EP-A 331 845, EP 554 651 or U.S. Pat. No. 4,187,383.

In general, the esterification may be carried out as follows:

The esterification apparatus comprises a stirred reactor, preferably areactor with circulatory evaporator and an added distillation unit withcondenser and phase separation vessel.

The reactor may be for example a reactor with jacketed heating and/orinternal heating coils. Preference is given to using a reactor having anexternal heat exchanger and natural or forced circulation, ie throughuse of a pump, more preferably natural circulation where circulation isaccomplished without mechanical aids.

It will be appreciated that the reaction can also be carried out in aplurality of reaction zones, for example a reactor battery of two tofour and preferably two or three reactors.

Suitable circulatory evaporators are known to one skilled in the art andare described for example in R. Billet, Verdampfertechnik, HTB-Verlag,Bibliographisches Institut Mannheim, 1965, 53. Examples of circulatoryevaporators are tube-bundle heat exchangers, plate-type heat exchangers,etc.

It will be appreciated that the circulatory system may also include aplurality of heat exchangers.

The distillation unit is of conventional design. It may be a simpledistillation unit which if appropriate is equipped with a splash guardor it may be a rectification column. Suitable column internals includein principle all common internals, for example trays, structuredpackings and/or dumped packings. Preferred trays include bubble trays,sieve trays, valve trays, Thormann trays and/or dual-flow trays, whilepreferred dumped packings are those of rings, coils, saddles or braids.

In general, from 5 to 20 theoretical plates are sufficient.

The condenser and the separation vessel are of traditional design.

The (meth)acrylic acid and the alkoxylated trimethylolpropane aregenerally used in the esterification a) in a molar excess as indicatedabove. The excess used can be up to about 3000:1, if desired.

Useful esterification catalysts C include those recited above.

They are generally used in an amount of 0.1-5% by weight, based on theesterification mixture, preferably 0.5-5%, more preferably 1-4% and mostpreferably 2-4% by weight.

If necessary, the esterification catalyst can be removed from thereaction mixture with the aid of an ion exchanger. The ion exchanger canbe added directly to the reaction mixture and then subsequently filteredoff, or the reaction mixture can be passed through an ion exchanger bed.

Preferably, the esterification catalyst is left in the reaction mixture.However, where the catalyst is an ion exchanger, the ion exchanger ispreferably removed, for example by filtration.

Stabilization may be further supported by the presence of anoxygen-containing gas, preferably air or a mixture of air and nitrogen(lean air).

This oxygen-containing gas is preferably metered into the bottom regionof a column and/or into a circulatory evaporator and/or passed throughand/or over the reaction mixture.

The polymerization inhibitor (mixture) D (as indicated above) is used ina total amount of 0.01-1% by weight, based on the esterificationmixture, preferably 0.02-0.8% and more preferably 0.05-0.5% by weight.

The polymerization inhibitor (mixture) D may be used for example as anaqueous solution or as a solution in a reactant or product.

b) The water of reaction formed in the course of the reaction can bedistilled off during or after the esterification a), in which case thisoperation can be augmented by a solvent which forms an azeotrope withwater.

Useful solvents E for azeotropic removal of the water of reaction, ifdesired, include the compounds recited above.

The esterification is preferably carried out in the presence of asolvent.

The amount of solvent used is 10-200% by weight, preferably 20-100% byweight and more preferably from 30% to 100% by weight, based on the sumtotal of alkoxylated trimethylolpropane and (meth)acrylic acid.

However, an operation without entrainer is also conceivable, asdescribed for example in DE-A1 38 43 854, column 2 line 18 to column 4line 45, but in contradistinction to the cited reference with theabovementioned stabilizers.

When the water in the reaction mixture is not removed via anazeotrope-forming solvent, it may be removed by stripping with an inertgas, preferably an oxygen-containing gas and more preferably air or leanair as described for example in DE-A 38 43 843.

The reaction temperature for the esterification a) is generally in therange from 40 to 160° C., preferably in the range from 60 to 140° C. andmore preferably in the range from 80 to 120° C. The temperature mayremain constant or rise in the course of the reaction and preferably itis raised in the course of the reaction. In this case, the finaltemperature of the esterification is 5-30° C. higher than the initialtemperature. The temperature of the esterification can be determined andcontrolled by varying the solvent concentration in the reaction mixture,as described in DE-A 199 41 136 and the German application under filereference 100 63 175.4.

When a solvent is used, it can be distilled out of the reaction mixturethrough the distillation unit added on top of the reactor.

The distillate may selectively be removed or, after condensation, fedinto a phase separation apparatus. The aqueous phase thus obtained isgenerally removed from the system, while the organic phase can be fed asreflux into the distillation unit and/or passed directly into thereaction zone and/or fed into a circulatory evaporator as described inthe German patent application under file reference 100 63 175.4.

When used as reflux, the organic phase can be used as described in DE-A199 41 136 for controlling the temperature in the esterification.

The esterification a) can be carried out with no pressure, atsuperatmospheric or reduced pressure and is preferably carried out atatmospheric pressure.

The reaction time is generally in the range from 2 to 20 hours,preferably in the range from 4 to 15 hours and more preferably in therange from 7 to 12 hours.

The order in which the individual reaction components are added is notessential to the present invention. All components can be introduced asa mixed initial charge and subsequently heated, or one or morecomponents may be omitted from or only partly included in the initialcharge and added only after the initial charge has been heated up.

The (meth)acrylic acid which can be used is not restricted in itscomposition and may comprise for example the following components:

(Meth)acrylic acid 90–99.9% by weight Acetic acid 0.05–3% by weightPropionic acid 0.01–1% by weight Diacrylic acid 0.01–5% by weight Water0.05–5% by weight Carbonylics 0.01–0.3% by weight Inhibitors 0.01–0.1%by weight Maleic acid or anhydride 0.001–0.5% by weight

The crude (meth)acrylic acid used is generally stabilized with 200-600ppm of phenothiazine or other stabilizers in amounts which permitcomparable stabilization. Carbonylics here refers for example to acetoneand lower aldehydes, for example formaldehyde, acetaldehyde,crotonaldehyde, acrolein, 2-furfural, 3-furfural and benzaldehyde.

Crude (meth)acrylic acid here refers to the (meth)acrylic acid mixturewhich is obtained after absorption of the reaction gases of thepropane/propene/acrolein or isobutane/isobutene/methacrolein oxidationin an absorbent and subsequent removal of the absorbent, or which isobtained by fractional condensation of the reaction gases.

It is obviously also possible to use pure (meth)acrylic acid, forexample of the following purity:

(Meth)acrylic acid 99.7–99.99% by weight Acetic acid 50–1000 weight ppmPropionic acid 10–500 weight ppm Diacrylic acid 10–500 weight ppm Water50–1000 weight ppm Carbonylics 1–500 weight ppm Inhibitors 1–300 weightppm Maleic acid or anhydride 1–200 weight ppm

The pure (meth)acrylic acid used is generally stabilized with 100-300ppm of hydroquinone monomethyl ether or other storage stabilizers inamounts which permit comparable stabilization.

Pure or prepurified (meth)acrylic acid generally refers to (meth)acrylicacid whose purity is at least 99.5% by weight and which is substantiallyfree of aldehydic, other carbonylic and high-boiling components.

The aqueous phase, distilled off during the esterification, of thecondensate removed via the added column (if present) may generallycontain 0.1-10% by weight of (meth)acrylic acid, and is separated offand removed from the system. The (meth)acrylic acid it contains maypreferably be extracted with an extractant, preferably with any solventused in the esterification, for example with cyclohexane, at from 10 to40° C. and a ratio of 1:5-30 and preferably 1:10-20 for aqueous phase toextractant, and returned into the esterification.

Circulation may be further supported by passing an inert gas, preferablyan oxygen-containing gas, more preferably air or a mixture of air andnitrogen (lean air) into the circulation or through or over the reactionmixture, for example at rates of 0.1-1, preferably 0.2-0.8 and morepreferably 0.3-0.7 m³/m³h, based on the volume of the reaction mixture.

The course of the esterification a) can be monitored by monitoring theamount of water carried out and/or the decrease in the carboxylic acidconcentration in the reactor.

The reaction can be ended for example as soon as 90%, preferably atleast 95% and more preferably at least 98% of the theoretically expectedamount of water has been carried out by the solvent.

The end of the reaction can be detected for example from the fact thatsubstantially no further water of reaction is removed via the entrainer.When (meth)acrylic acid is carried out together with the water ofreaction, its fraction is determinable for example by backtitrating analiquot of the aqueous phase.

The removal of the water of reaction can be dispensed with for examplewhen the (meth)acrylic acid is used in a high stoichiometric excess, forexample of at least 4.5:1, preferably at least 7.5:1 and most preferablyat least 15:1. In this case, a substantial portion of the amount ofwater formed will remain in the reaction mixture. Merely that fractionof water is removed from the reaction mixture during or after thereaction which is determined by the volatility at the employedtemperature and beyond that no measures are carried out to remove theresulting water of reaction. For instance, at least 10% by weight of theresulting water of reaction can remain in the reaction mixture,preferably at least 20% by weight, more preferably at least 30% byweight, even more preferably at least 40% by weight and most preferablyat least 50% by weight.

c) After the end of the esterification the reaction mixture can beconventionally cooled to 10-30° C. and if necessary by addition of asolvent which may be the same as any solvent used for azeotropic removalof water or a different solvent adjusted to any desired target esterconcentration.

In a further embodiment, the reaction can be stopped with a suitablediluent G and diluted to a concentration of for example 10-90% byweight, preferably 20-80%, more preferably 20-60%, even more preferably30-50% and most preferably about 40%, for example in order to reduce theviscosity.

What is important is that a substantially homogeneous solution formsafter dilution.

This is preferably accomplished only relatively shortly before use inthe production of the hydrogel, for example not more than 24 hoursbefore, preferably not more than 20 hours before, more preferably notmore than 12 hours before, even more preferably not more than 6 hoursbefore and most preferably not more than 3 hours before.

The diluent G is selected from the group consisting of water, a mixtureof water with one or more organic solvents which are soluble in water inany proportion and a mixture of water with one or more monohydric orpolyhydric alcohols, for example methanol and glycerol. The alcoholspreferably bear 1, 2 or 3 hydroxyl groups and preferably have from 1 to10 and especially up to 4 carbon atoms. Preference is given to primaryand secondary alcohols.

Preferred alcohols are methanol, ethanol, isopropanol, ethylene glycol,glycerol, 1,2-propanediol and 1,3-propanediol.

d) If necessary, the reaction mixture may be decolorized, for example bytreatment with active carbon or metal oxides, for example alumina,silica, magnesium oxide, zirconium oxide, boron oxide or mixturesthereof, in amounts for example of 0.1-50% by weight, preferably from0.5% to 25% by weight, more preferably 1-10% by weight at temperaturesof for example from 10 to 100° C., preferably from 20 to 80° C. and morepreferably from 30 to 60° C.

This can be effected by adding the pulverulent or granular decolorizingagent to the reaction mixture and subsequent filtration or by passingthe reaction mixture through a bed of the decolorizing agent in the formof any desired suitable moldings.

The decolorizing of the reaction mixture can be effected at any desiredstage in the workup process, for example at the stage of the crudereaction mixture or after any prewash, neutralization, wash or solventremoval.

The reaction mixture can further be subjected to a prewash e) and/or aneutralization f) and/or an afterwash g), preferably merely to aneutralization f). If desired, a neutralization f) and a prewash e) canbe interchanged in the sequence.

(Meth)acrylic acid, and/or catalyst C can be at least partly recoveredfrom the aqueous phase of the washes e) and g) and/or neutralization f)by acidification and extraction with a solvent and reused.

For a pre- or afterwash e) or g), the reaction mixture is treated in awash apparatus with a wash liquor, for example water or a 5-30% byweight, preferably 5-20% and more preferably 5-15% by weight sodiumchloride, potassium chloride, ammonium chloride, sodium sulfate orammonium sulfate solution, preferably water or sodium chloride solution.

The ratio of reaction mixture to wash liquor is generally in the rangefrom 1:0.1 to 1:1, preferably in the range from 1:0.2 to 1:0.8 and morepreferably in the range from 1:0.3 to 1:0.7.

The wash or neutralization can be carried out for example in a stirredcontainer or in other conventional apparatuses for example in a columnor a mixer-settler apparatus.

In terms of process engineering, any wash or neutralization in theprocess according to the present invention can be carried out usingconventional extraction and washing processes and apparatuses, forexample those described in Ullmann's Encyclopedia of IndustrialChemistry, 6th ed, 1999 Electronic Release, Chapter: Liquid—LiquidExtraction—Apparatus. For example, the choice may be for single- ormulti-staged, preferably single-staged, extractions, and also for thesein cocurrent or countercurrent mode and preferably in countercurrentmode.

Preference is given to using sieve tray columns, arrangedly or randomlypacked columns, stirred vessels or mixer-settler apparatuses and alsopulsed columns or columns having rotating internals.

The prewash e) is preferably used whenever metal salts and preferablycopper or copper salts are (concomitantly) used as inhibitors.

An afterwash g) may be preferable to remove traces of base or salttraces from the reaction mixture neutralized in f).

By way of neutralization f), the reaction mixture which may have beenprewashed and which may still contain small amounts of catalyst and themain amount of excess (meth)acrylic acid can be neutralized with a5-25%, preferably 5-20% and more preferably 5-15% by weight aqueoussolution of a base, for example alkali metal or alkaline earth metaloxides, hydroxides, carbonates or bicarbonates, preferably aqueoussodium hydroxide solution, aqueous potassium hydroxide solution, sodiumbicarbonate, sodium carbonate, potassium bicarbonate, calcium hydroxide,milk of lime, ammonia gas, ammonia water or potassium carbonate, towhich solution 5-15% by weight of sodium chloride, potassium chloride,ammonium chloride or ammonium sulfate may have been added, if desired,more preferably with aqueous sodium hydroxide solution or aqueous sodiumhydroxide-sodium chloride solution. The degree of neutralization ispreferably in the range from 5 to 60 mol %, preferably in the range from10 to 40 mol %, more preferably in the range from 20 to 30 mol %, basedon the acid-functional monomers. This neutralization can take placebefore and/or during the polymerization, preferably before thepolymerization.

The base is added in such a way that the temperature in the apparatusdoes not rise above 60° C. and is preferably in the range from 20 to 35°C., and the pH is 4-13. The heat of neutralization is preferably removedby cooling the vessel with the aid of internal cooling coils or viajacketed cooling.

The ratio of reaction mixture to neutralizing liquor is generally in therange from 1:0.1 to 1:1, preferably in the range from 1:0.2 to 1:0.8 andmore preferably in the range from 1:0.3 to 1:0.7.

With regard to the apparatus, the above statements apply.

h) When a solvent is present in the reaction mixture, it may besubstantially removed by distillation. Preferably, any solvent presentis removed from the reaction mixture after washing and/orneutralization, but if desired this may also be done prior to the washor neutralization.

For this, the reaction mixture is admixed with an amount of storagestabilizer, preferably hydroquinone monomethyl ether, such that, afterremoval of the solvent, 100-500, preferably 200-500 and more preferably200-400 ppm thereof are present in the target ester (residue).

The distillative removal of the main amount of solvent is effected forexample in a stirred tank with jacketed heating and/or internal heatingcoils under reduced pressure, for example at 20-700 mbar, preferably30-500 mbar and more preferably 50-150 mbar and 40-80° C.

It will be appreciated that the distillation can also be accomplished ina falling-film or thin-film evaporator. For this, the reaction mixtureis recirculated, preferably two or more times, through the apparatusunder reduced pressure, for example at 20-700 mbar, preferably 30-500mbar and more preferably 50-150 mbar and 40-80° C.

An inert gas, preferably an oxygen-containing gas, more preferably airor a mixture of air and nitrogen (lean air) may preferably be introducedinto the distillation apparatus, for example 0.1-1, preferably 0.2-0.8and more preferably 0.3-0.7 m³/m³h, based on the volume of the reactionmixture.

The residual solvent content of the residue is generally below 5% byweight, preferably 0.5-5% and more preferably 1-3% by weight after thedistillation.

The removed solvent is condensed and preferably reused.

If necessary, a solvent stripping operation i) can be carried out inaddition to or in lieu of the distillation.

For this, the target ester, which still contains small amounts ofsolvent, is heated to 50-90° C. and preferably 80-90° C. and theremaining amounts of solvent are removed with a suitable gas in asuitable apparatus. There are circumstances where a vacuum can beapplied in support, if desired.

Examples of useful apparatus include columns of conventional designwhich contain conventional internals, for example trays, dumped packingor structured packing, preferably dumped packing. Useful columninternals include in principle all common internals, for example trays,arranged packing and/or random packing. Preferred trays include bubbletrays, sieve trays, valve trays, Thormann trays and/or dual-flow trays,while preferred dumped packings are those of rings, coils, saddles,Raschig, Intos or Pall rings, barrel or Intalox saddles, Top-Pak, etc orbraids.

Another possibility here is a falling-film, thin-film or wipe-filmevaporator, for example a Luwa, Rotafilm or Sambay evaporator, which maybe splash-guarded with a demister for example.

Useful gases include gases which are inert under the strippingconditions, preferably oxygen-containing gases, more preferably air ormixtures of air and nitrogen (lean air) or water vapor, especially suchgases which have been preheated to 50-100° C.

The stripping gas rate is for example in the range from 5 to 20, morepreferably in the range from 10 to 20 and most preferably in the rangefrom 10 to 15 m³/m³h, based on the volume of the reaction mixture.

If necessary, the ester can be subjected to a filtration j) at any stageof the workup process, preferably after washing/neutralization and anyeffected solvent removal, in order that precipitated traces of salts andany decolorizing agent may be removed.

In a conceivable embodiment, the esterification a) of alkoxylatedtrimethylolpropane with (meth)acrylic acid in the presence of at leastone esterification catalyst C and of at least one polymerizationinhibitor D is carried out in a molar excess of at least 15:1, asindicated above, without a solvent capable of forming an azeotrope withwater.

In a preferred embodiment the excess (meth)acrylic acid is preferablysubstantially not removed, ie only that fraction of (meth)acrylic acidis removed from the reaction mixture that is determined by thevolatility at the employed temperature, and beyond that no measures arecarried out to remove the carboxylic acid, for example no distillative,rectificative, extractive (washing for example), absorptive (for examplepassing through activated carbon or through ion exchangers) and/orchemical steps such as scavenging of the carboxylic acid with epoxidesare carried out.

The extent to which the (meth)acrylic acid in the reaction mixture isremoved from it is preferably not more than 75% by weight, morepreferably not more than 50% by weight, even more preferably not morethan 25% by weight, especially not more than 10% by weight and mostpreferably not more than 5% by weight, based on the (meth)acrylic acidin the reaction mixture after the reaction has ended. In a particularlypreferred embodiment, stage b) can be omitted, so that only the fractionof water of reaction and (meth)acrylic acid is removed from the reactionmixture that is determined by the volatility at the employedtemperature. This can preferably be prevented by substantially completecondensation.

Furthermore, the esterification catalyst C used is likewisesubstantially left in the reaction mixture.

The DIN EN 3682 acid number of the reaction mixture thus obtainable ispreferably at least 25 mg of KOH/g of reaction mixture, more preferablyin the range from 25 to 80 and most preferably in the range from 25 to50 mg of KOH/g.

Any pre- or afterwash e) or g) is preferably omitted; merely afiltration step j) can be sensible.

The reaction mixture can subsequently be diluted in step c), in whichcase it is preferably converted within 6 hours and more preferablywithin 3 hours to the hydrogel. It may preferably be neutralized in astep f).

The order of the steps c), j) and f) is arbitrary.

The present invention further provides a composition of mattercomprising

-   -   at least one ester F obtainable by one of the esterification        processes described above,    -   (meth)acrylic acid and    -   diluent G.

The composition of matter of the present invention may further comprise

-   -   esterification catalyst C in protonated or unprotonated form,    -   polymerization inhibitor D and also    -   any solvent E if used in the esterification.

The composition of matter may have been neutralized and have a pH ascited above under f).

When the composition of matter has been neutralized, at least a portionof the (meth)acrylic acid has been converted into their water-solublealkali metal, alkaline earth metal or ammonium salts.

A preferred composition of matter comprises

-   -   ester F in a fraction from 0.1% to 40% by weight, more        preferably from 0.5% to 20%, even more preferably from 1% to        10%, especially from 2% to 5% and specifically from 2% to 4% by        weight,    -   monomer M at 0.5-99.9% by weight, more preferably 0.5-50% by        weight, even more preferably 1-25%, especially 2-15% and        specifically from 3% to 5% by weight,    -   esterification catalyst C at 0-10% by weight, more preferably        0.02-5%, even more preferably 0.05-2.5% by weight and especially        0.1-1% by weight,    -   polymerization inhibitor D at 0-5% by weight, more preferably        0.01-1.0%, even more preferably 0.02-0.75%, especially 0.05-0.5%        and specifically 0.075-0.25% by weight,    -   solvent E at 0-10% by weight, more preferably 0-5% by weight,        even more preferably 0.05-1.5% by weight and especially 0.1-0.5%        by weight, with the proviso that the sum total is always 100% by        weight, and also    -   any diluent G ad 100% by weight.

The reaction mixtures obtainable by the above process and compositionsof matter according to the present invention can find use

-   -   as a radical crosslinker of water-absorbing hydrogels,    -   as a starting material for producing polymer dispersions,    -   as a starting material for producing polyacrylates (except        hydrogels),    -   as a paint raw material or    -   as a cement additive.

Compositions of matter according to the present invention which areparticularly useful as radical crosslinkers of water-absorbing hydrogelshave a solubility in distilled water at 25° C. of not less than 0.5% byweight, preferably not less than 1% by weight, more preferably not lessthan 2% by weight, even more preferably not less than 5% by weight,still more preferably not less than 10% by weight, yet even morepreferably not less than 20% by weight and especially not less than 30%by weight.

k) The reaction mixture from the esterification, including workup stepsthereof, where practiced, for example the reaction mixture from f) or,when f) is omitted, from b) or, when b) is omitted, the reaction mixturefrom a), can optionally be admixed with additional monoethylenicallyunsaturated compounds N which bear no acid groups but arecopolymerizable with the hydrophilic monomers M and can then bepolymerized in the presence of at least one radical initiator K andoptionally at least one grafting base L to prepare water-absorbinghydrogels.It may be preferablel) to postcrosslink the reaction mixture of k).

Useful hydrophilic monomers M for preparing k) these highly swellablehydrophilic hydrogels include for example acids capable of additionpolymerization, such as acrylic acid, methacrylic acid, ethacrylic acid,α-chloroacrylic acid, crotonic acid, maleic acid, maleic anhydride,vinylsulfonic acid, vinylphosphonic acid, maleic acid, maleic anhydride,fumaric acid, itaconic acid, citraconic acid, mesaconic acid, glutaconicacid, aconitic acid, allylsulfonic acid, sulfoethyl acrylate, sulfoethylmethacrylate, sulfopropyl acrylate, sulfopropyl methacrylate,2-hydroxy-3-acryioyloxypropylsulfonic acid,2-hydroxy-3-methacryloyloxypropylsulfonic acid, allylphosphonic acid,styrenesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid,2-acrylamido-2-methylpropanephosphonic acid and also their amides,hydroxyalkyl esters and amino- or ammonio-containing esters and amides.These monomers can be used alone or mixed with each other. Furthermorewater-soluble N-vinylamides and also diallyldimethylammonium chloride.Preferred hydrophilic monomers are compounds of the formula V

where

-   R³ is hydrogen, methyl or ethyl,-   R⁴ is —COOR⁶, a sulfonyl group, a phosphonyl group, a    (C₁-C₄)-alkanol-esterified phosphonyl group of the formula VI

-   R⁵ is hydrogen, methyl, ethyl or a carboxyl group,-   R⁶ is hydrogen, amino or hydroxy-(C₁-C₄)-alkyl and-   R⁷ is a sulfonyl group, a phosphonyl group or a carboxyl group.

Examples of (C₁-C₄)-alkanols are methanol, ethanol, n-propanol andn-butanol.

Particularly preferred hydrophilic monomers are acrylic acid andmethacrylic acid, especially acrylic acid.

To optimize properties, it can be sensible to use additionalmonoethylenically unsaturated compounds N which do not bear an acidgroup but are copolymerizable with the monomers bearing acid groups.Such compounds include for example the amides and nitrites ofmonoethylenically unsaturated carboxylic acid, for example acrylamide,methacrylamide and N-vinylformamide, N-vinylacetamide,N-methylvinyl-acetamide, acrylonitrile and methacrylonitrile. Examplesof further suitable compounds are vinyl esters of saturated C₁- toC₄-carboxylic acids such as vinyl formate, vinyl acetate or vinylpropionate, alkyl vinyl ethers having at least 2 carbon atoms in thealkyl group, for example ethyl vinyl ether or butyl vinyl ether, estersof monoethylenically unsaturated C₃- to C₆-carboxylic acids, for exampleesters of monohydric C₁- to C₁₈-alcohols and acrylic acid, methacrylicacid or maleic acid, monoesters of maleic acid, for example methylhydrogen maleate, N-vinyllactams such as N-vinylpyrrolidone orN-vinylcaprolactam, acrylic and methacrylic esters of alkoxylatedmonohydric saturated alcohols, for example of alcohols having from 10 to25 carbon atoms which have been reacted with from 2 to 200 mol ofethylene oxide and/or propylene oxide per mole of alcohol, and alsomonoacrylic esters and monomethacrylic esters of polyethylene glycol orpolypropylene glycol, the molar masses (Mn) of the polyalkylene glycolsbeing up to 2000, for example. Further suitable monomers are styrene andalkyl-substituted styrenes such as ethylstyrene or tert-butylstyrene.

These monomers without acid groups may also be used in mixture withother monomers, for example mixtures of vinyl acetate and 2-hydroxyethylacrylate in any proportion. These monomers without acid groups are addedto the reaction mixture in amounts within the range from 0 to 50% byweight, preferably less than 20% by weight.

The crosslinked (co)polymers preferably consist of acid-functionalmonoethylenically unsaturated monomers which have optionally beenconverted into their alkali metal or ammonium salts before or afterpolymerization and of 0-40% by weight based on their total weight ofmonoethylenically unsaturated monomers which do not bear acid groups.

The production, testing and use of (meth)acrylic acid (co)polymers,polyacrylic acids and superabsorbents has been extensively describedbefore and therefore is well known, see for example “ModernSuperabsorbent Polymer Technology”, F. L. Buchholz and A. T. Graham,Wiley-VCH, 1998 or Markus Frank-“Superabsorbents” in Ullmann's Handbuchder technischen Chemie, Volume 35, 2003.

Preference is given to such hydrogels which are obtained by crosslinkingaddition polymerization or copolymerization of acid-functionalmonoethylenically unsaturated monomers M or salts thereof.

The polymers obtainable are notable for an improved saponification index(VSI).

In the postcrosslinking process, the starting polymer is treated with apostcrosslinker and preferably during or after the treatmentpostcrosslinked and dried by raising the temperature, the crosslinkerpreferably being included in an inert solvent. Inert solvents aresolvents which substantially do not react either with the startingpolymer or with the postcrosslinker. Preference is given to suchsolvents which do not react chemically with the starting polymer or withthe postcrosslinker to an extent of more than 90%, preferably more than95%, more preferably more than 99% and especially more than 99.5%.

Postcrosslinking l) and drying m) is preferably carried out at from 30to 250° C., especially 50-200° C. and most preferably at from 100 to180° C. The surface postcrosslinking solution is preferably applied byspraying the polymer in suitable spray mixers. After spraying, thepolymer powder is thermally dried, and the crosslinking reaction cantake place not only before but also during the drying operation.Preference is given to spraying a solution of the crosslinker inreaction mixers or mixing and drying ranges such as for example Lödigemixers, BEPEX mixers, NAUTA mixers, SHUGGI mixers or PROCESSALL. It ismoreover also possible to use fluidized bed dryers.

The drying operation can take place in the mixer itself, by heating ofthe shell or by blowing in hot air. Also suitable is a downstream dryersuch as for example a shelf dryer, a rotary tube oven or a heatablescrew. But it is also possible to utilize an azeotropic distillation asdrying technique, for example. The preferred residence time at thistemperature in the reaction mixer or dryer is below 60 min and morepreferably below 30 min.

Preference is given to the above processes wherein the starting polymeris a polymeric acrylic acid or a polyacrylate, especially a polymericacrylic acid or a polyacrylate obtained by free-radical polymerizationusing a polyfunctional ethylenically unsaturated radical crosslinker.

Preference is given to such processes wherein the composition of mattercontaining radical crosslinkers, ie the ester F, and diluents G in aratio of 0.1-20% by weight and especially 0.5-10% by weight based on themass of the starting polymer is used.

Preference is given to such processes wherein the radical crosslinker isused in a dose of 0.01-5.0% by weight, preferably 0.02-3.0% by weight,more preferably 0.03-2.5% by weight, especially 0.05-1.0% andspecifically from 0.1% to 0.75% by weight based on the starting polymer.

The present invention also provides polymers prepared by one of theprocesses mentioned above and for their use in hygiene articles,packaging materials and nonwovens and also for the use of anabovementioned composition of matter for producing crosslinked orthermally crosslinkable polymers, especially in paints and varnishes.

The highly swellable hydrophilic hydrogels to be used (startingpolymers) are in particular polymers of (co)polymerized hydrophilicmonomers M, graft (co)polymers of one or more hydrophilic monomers M ona suitable grafting base L, crosslinked cellulose or starch ethers ornatural products capable of swelling in aqueous fluids, for example guarderivatives. These hydrogels are known to one skilled in the art and aredescribed for example in U.S. Pat. No. 4,286,082, DE-C-27 06 135, U.S.Pat. No. 4,340,706, DE-C-37 13 601, DE-C-28 40 010, DE-A-43 44 548,DE-A-40 20 780, DE-A-40 15 085, DE-A-39 17 846, DE-A-38 07 289, DE-A-3533 337, DE-A-35 03 458, DE-A-42 44 548, DE-A-42 19 607, DE-A-40 21 847,DE-A-38 31 261, DE-A-35 11 086, DE-A-31 18 172, DE-A-30 28 043, DE-A-4418 881, EP-A-0 801 483, EP-A-0 455 985, EP-A-0 467 073, EP-A-0 312 952,EP-A-0 205 874, EP-A-0 499 774, DE-A 26 12 846, DE-A-40 20 780, EP-A-020 5674, U.S. Pat. No. 5,145,906, EP-A-0 530 438, EP-A-0 670 073, U.S.Pat. No. 4,057,521, U.S. Pat. No. 4,062,817, U.S. Pat. No. 4,525,527,U.S. Pat. No. 4,295,987, U.S. Pat. No. 5,011,892, U.S. Pat. No.4,076,663, U.S. Pat. No. 4,931,497. Also of particular suitability arehighly swellable hydrogels from a manufacturing operation as describedin WO 01/38402 and also highly swellable inorganic/organic hybridhydrogels as described in DE 198 54 575. The content of theaforementioned patent documents, especially the hydrogels obtained bythe processes, is incorporated herein by reference.

Suitable grafting bases L for hydrophilic hydrogels obtainable by graftcopolymerization of olefinically unsaturated acids can be of natural orsynthetic origin. Examples are starch, cellulose, cellulose derivativesand also other polysaccharides and oligosaccharides, polyalkyleneoxides, especially polyethylene oxides and polypropylene oxides, andalso hydrophilic polyesters.

The water-absorbing polymer is obtainable by free-radical graftcopolymerization of acrylic acid or acrylate onto a water-solublepolymer matrix. Nonlimiting examples of suitable water-soluble polymermatrices are alginates, polyvinyl alcohol and polysaccharides such asstarch for example. A graft copolymerization for the purposes of thepresent invention utilizes a polyfunctional ethylenically unsaturatedradical crosslinker.

The water-absorbing polymer can be an organic/inorganic hybrid polymerformed from a polymeric acrylic acid or polyacrylate on the one hand anda silicate, aluminate or aluminosilicate on the other. Moreparticularly, the polymeric acrylic acid or polyacrylate used may beobtained by free-radical polymerization using a polyfunctionalethylenically unsaturated radical crosslinker and formed using awater-soluble silicate or soluble aluminate or mixture thereof.

Preferred hydrogels are in particular polyacrylates, polymethacrylatesand also the U.S. Pat. No. 4,931,497, U.S. Pat. No. 5,011,892 and U.S.Pat. No. 5,041,496 graft polymers. Very particularly preferred hydrogelsare the kneader polymers described in WO 01/38402 and thepolyacrylate-based organic/inorganic hybrid hydrogels described in DE198 545 75.

The substances prepared according to the present invention, which areuseful as radical crosslinkers in hydrogels, can be used alone or incombination with other crosslinkers, for example internal or surfacecrosslinkers, for example the following:

Suitable crosslinkers are in particular methylenebisacrylamide,methylenebismethacrylamide, esters of unsaturated mono- orpolycarboxylic acids with polyols, such as diacrylate or triacrylate,for example butanediol diacrylate, butanediol dimethacrylate, ethyleneglycol diacrylate, ethylene glycol dimethacrylate, and alsotrimethylolpropane triacrylate and allyl compounds such as allyl(meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters,tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allylesters of phosphoric acid and also vinylphosphonic acid derivatives asdescribed for example in EP-A-0 343 427. Suitable crosslinkers arepentaerythritol triallyl ether, pentaerythritol tetraallyl ether,polyethylene glycol diallyl ether, monoethylene glycol diallyl ether,glycerol diallyl ether, glycerol triallyl ether, polyallyl ethers basedon sorbitol and also ethoxylated variants thereof. Particularlypreferred crosslinkers further include polyethylene glycol diacrylates,ethoxylated derivatives of trimethylolpropane triacrylate, for exampleSartomer SR 9035, and also ethoxylated derivatives of glyceroldiacrylate and glycerol triacrylate. It is obviously also possible touse mixtures of the above crosslinkers.

Very particular preference is given to hydrogels prepared using an esterF prepared according to the present invention as a radical crosslinker.

The water-absorbing polymer is preferably a polymeric acrylic acid or apolyacrylate. This water-absorbing polymer can be prepared by a processknown from the literature. Preference is given to polymers which containcrosslinking comonomers (0.001-10 mol %), but very particular preferenceis given to polymers which were obtained by free-radical polymerizationand where a polyfunctional ethylenically unsaturated radical crosslinkerwas used.

The highly swellable hydrophilic-hydrogels are preparable by additionpolymerization processes known per se. Preference is given to theaddition polymerization in aqueous solution conducted as a gelpolymerization. It involves, as stated above, dilute, preferably aqueousand more preferably 15-50% by weight aqueous, solutions of one or morehydrophilic monomers and optionally of a suitable grafting base L beingpolymerized in the presence of a free radical initiator by utilizing theTrommsdorff-Norrish effect (Makromol. Chem. 1, 169 (1947)) preferablywithout mechanical mixing. The polymerization reaction may be carriedout at from 0° C. to 150° C., and preferably at from 10° C. to 100° C.,not only at atmospheric pressure but also at superatmospheric or reducedpressure. Typically, the polymerization can also be carried out in aprotective gas atmosphere, preferably under nitrogen. The additionpolymerization may be induced using high-energy electromagnetic rays orthe customary chemical polymerization initiators K, for example organicperoxides, such as benzoyl peroxide, tert-butyl hydroperoxide, methylethyl ketone peroxide, cumene hydroperoxide, azo compounds such asazobisisobutyronitrile and also inorganic peroxy compounds such as(NH₄)₂S₂O₈, K₂S₂O₈ or H₂O₂.

They can if desired be used in combination with reducing agents such asascorbic acid, sodium hydrogensulfite and iron(II) sulfate or redoxsystems where the reducing component included is an aliphatic andaromatic sulfinic acid, such as benzenesulfinic acid and toluenesulfinic acid or derivatives thereof, for example Mannich adducts ofsulfinic acids, aldehydes and amino compounds, as described in DE-C-1301 566. The performance properties of the polymers can be furtherimproved by postheating the polymer gels in the temperature range from50° to 130° C. and preferably from 700 to 100° C. for several hours.

The gels obtained are neutralized to the extent of 0-100 mol %,preferably 25-100 mol % and more preferably 50-85 mol % based on monomerused, for which the customary neutralizing agents can be used,preferably alkali metal hydroxides, alkali metal oxides or thecorresponding alkali metal carbonates, but more preferably sodiumhydroxide, sodium carbonate and sodium bicarbonate.

Neutralization is typically achieved by mixing the neutralizing agent asan aqueous solution or else preferably as a solid into the gel. Forthis, the gel is mechanically comminuted, for example by means of a meatgrinder, and the neutralizing agent is sprayed on, scattered on orpoured on and then carefully mixed in. The gel mass obtained can then berepeatedly passed through the meat grinder for homogenization. Theneutralized gel mass is then dried with a belt or can dryer until theresidual moisture content is preferably below 10% by weight andespecially below 5% by weight.

The addition polymerization as such can also be carried out by any otherprocess described in the literature. More particularly, theneutralization of the acrylic acid can also be carried out prior to thepolymerization, as described above in step f). The polymerization canthen be carried out in a conventional belt reactor or a kneading reactorcontinuously or else batchwise. When the polymerization is carried outin a belt reactor, initiation by electromagnetic radiation andpreferably by UV radiation or alternatively initiation by means of aredox initiator system is particularly preferred. Very particularpreference is also given to a combination of the two methods ofinitiation: electromagnetic radiation and chemical redox initiatorsystem simultaneously.

n) The dried hydrogel can then be ground and sieved, in which case it iscustomary to use roll mills, pin mills or vibratory mills for thegrinding. The preferred particle size of the sieved hydrogel ispreferably in the range 45-1000 μm, more preferably at 45-850 μm, evenmore preferably at 200-850 μm, and most preferably at 300-850 μm, andparticular preference is also given to the range from 150 to 850 μm, andvery particularly to the range from 150 to 700 μm. These rangespreferably cover 80% by weight of the particles and especially 90% byweight of the particles. The size distribution can be determined usingestablished laser methods.

The present invention further provides crosslinked hydrogels whichcontain at least one hydrophilic monomer M in copolymerized form andhave been crosslinked using an ester F of alkoxylated trimethyolpropanewith (meth)acrylic acid. The ester can be prepared in a manner accordingto the present invention or in a prior art manner and is preferablyprepared in a manner according to the present invention.

Useful esters F include compounds as described above.

The CRC value [g/g] of the hydrogel-forming polymers according to thepresent invention can be measured by the methods indicated in thedescription and is preferably above 15, especially 16, 18, 20, 22, 24,or higher, more preferably 25, especially 26, 27, 28, 29, even morepreferably 30, 31, 32, 33, 34, 35, 36, 37 or higher.

The AUL 0.7 psi value [g/g] of the hydrogel-forming polymers accordingto the present invention can be measured by the methods indicated in thedescription part and is preferably above 8, especially 9, 10, 11, 12,13, 14 or higher, more preferably 15, especially 16, 17, 18, 19, orhigher, even more preferably above 20, especially 21, 22, 23, 24, 25,26, 27, 28, or higher.

The AUL 0.5 psi value [g/g] of the hydrogel-forming polymers accordingto the present invention can be measured by the methods indicated in thedescription part and is preferably above 8, especially 9, 10, 11, 12,13, 14 or higher, more preferably 15, especially 16, 17, 18, 19, orhigher, even more preferably above 20, especially 21, 22, 23, 24, 25,26, 27, 28, or higher.

The saponification index VSI of the hydrogel-forming polymers accordingto the present invention can be measured by the methods indicated in thedescription part and is preferably less than 10, especially 9.5, 9 or8.5 or lower, more preferably less than 8, especially 7.5, 7, 6.5, 6,5.5 or lower, even more preferably less than 5, especially 4.4, 4 orlower.

Application and use of the hydrogel-forming polymers according to thepresent invention

The present invention further relates to the use of the abovementionedhydrogel-forming polymers in hygiene articles comprising

-   (P) a liquid-pervious topsheet-   (Q) a liquid-impervious backsheet-   (R) a core positioned between (P) and (O) and comprising-   10-100% by weight of the hydrogel-forming polymer according to the    present invention 0-90% by weight of hydrophilic fiber material-   preferably 20-100% by weight of the hydrogel-forming polymer    according to the present invention, 0-80% by weight of hydrophilic    fiber material-   more preferably 30-100% by weight of the hydrogel-forming polymer    according to the present invention, 0-70% by weight of hydrophilic    fiber material-   even more preferably 40-100% by weight of the hydrogel-forming    polymer according to the present invention, 0-60% by weight of    hydrophilic fiber material-   yet even more preferably 50-100% by weight of the hydrogel-forming    polymer according to the present invention, 0-50% by weight of    hydrophilic fiber material-   particularly preferably 60-100% by weight of the hydrogel-forming    polymer according to the present invention, 0-40% by weight of    hydrophilic fiber material-   especially preferably 70-100% by weight of the hydrogel-forming    polymer according to the present invention, 0-30% by weight of    hydrophilic fiber material-   extremely preferably 80-100% by weight of the hydrogel-forming    polymer according to the present invention, 0-20% by weight of    hydrophilic fiber material-   most preferably 90-100% by weight of the hydrogel-forming polymer    according to the present invention, 0-10% by weight of hydrophilic    fiber material-   (S) optionally a tissue layer positioned directly above and below    said core (R), and-   (T) optionally an acquisition layer positioned between (P) and (R).

The percentages are to be understood so that in the case of 10-100% byweight, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% up to in each case100% by weight of hydrogel-forming polymer according to the presentinvention and all intermediate % (for example 12.2%) are possible andcorrespondingly hydrophilic fiber material from 0% to in each case 89%,88%, 87%, 86%, 85%, 83%, 82%, 81% by weight and intermediate percentages(for example 87.8%) are possible. When further materials are present inthe core, the percentages of polymer and fiber decrease accordingly. Thesame applies to the preferred ranges, for example in the case ofextremely preferable 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89% byweight can be present for the hydrogel-forming polymer according to thepresent invention and correspondingly 19%, 18%, 17%, 16%, 15%, 14%, 13%,12%, 11% by weight for the fiber material. Thus, 20%, 21%, 22%, 23%,24%, 25%, 26%, 27%, 28%, 29% to 100% by weight of the hydrogel-formingpolymer according to the present invention can be present in thepreferred range, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39% to100% by weight can be present for the hydrogel-forming polymer accordingto the present invention, in the more preferred range, 40%, 41%, 42%,43%, 44%, 45%, 46%, 47%, 48%, 49% to 100% by weight can be present forthe hydrogel-forming polymer according to the present invention, in theeven more preferred range, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%,59% to 100% by weight can be present for the hydrogel-forming polymeraccording to the present invention, in the yet even more preferredrange, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69% to 100% byweight can be present for the hydrogel-forming polymer according to thepresent invention, in the particularly preferred range, 70%, 71%, 71%,72%, 73%, 74%, 75%, 76%, 77%, 78%, 79% to 100% by weight can be presentfor the hydrogel-forming polymer according to the present invention inthe especially preferred range, and 90%, 91%, 92%, 93%, 94%, 95%, 96%,97%, 98%, 99% or 100% by weight can be present for the hydrogel-formingpolymer according to the present invention in the most preferred range.

Hygiene articles for the purposes of the present invention include notonly incontinence pads and incontinence briefs for adults but alsodiapers for infants.

The liquid-pervious topsheet (P) is the layer which is in direct contactwith the skin of the wearer. Its material comprises customary syntheticor manufactured fibers or films of polyesters, polyolefins, rayon ornatural fibers such as cotton. In the case of non-woven materials thefibers are generally joined together by binders such as polyacrylates.Preferred materials are polyesters, rayon and blends thereof,polyethylene and polypropylene. Examples of liquid-pervious layers aredescribed in WO 99/57355 A1, EP 102 388 3 A2.

The liquid-impervious layer (O) is generally a sheet of polyethylene orpolypropylene.

The core (R) includes not only the hydrogel-forming polymer according tothe present invention but also hydrophilic fiber material. Byhydrophilic is meant that aqueous fluids spread quickly over the fiber.The fiber material is usually cellulose, modified cellulose, rayon,polyester such as polyethylene terephthalate. Particular preference isgiven to cellulose fibers such as pulp. The fibers generally have adiameter of 1-200 μm and preferably 10-100 μm, and also have a minimumlength of 1 mm.

Diaper construction and shape is common knowledge and described forexample in WO 95/26 209 page 66 line 34 to page 69 line 11, DE 196 04601 A1, EP-A-0 316 518 and EP-A-0 202 127. Diapers and other hygienearticles are generally also described in WO 00/65084, especially atpages 6-15, WO 00/65348, especially at pages 4-17, WO 00/35502,especially pages 3-9, DE 19737434, WO 98/8439. Hygiene articles forfeminine care are described in the following references. The subjecthydrogel-forming polymers capable of absorbing aqueous fluids can beused there. Feminine care references: WO 95/24173: Absorption Articlefor Controlling Odour, WO 91/11977: Body Fluid Odour Control, EP 389023:Absorbent Sanitary Articles, WO 94/25077: Odour Control Material, WO97/01317: Absorbent Hygienic Article, WO 99/18905, EP 834297, U.S. Pat.No. 5,762,644, U.S. Pat. No. 5,895,381, WO 98/57609, WO 2000/065083, WO2000/069485, WO 2000/069484, WO 2000/069481, U.S. Pat. No. 6,123,693, EP1104666, WO 2001/024755, WO 2001/000115, EP 105373, WO 2001/041692, EP1074233. Tampons are described in the following references: WO 98/48753,WO 98/41179, WO 97/09022, WO 98/46182, WO 98/46181, WO 2001/043679, WO2001/043680, WO 2000/061052, EP 1108408, WO 2001/033962, DE 200020662,WO 2001/001910, WO 2001/001908, WO 2001/001909, WO 2001/001906, WO2001/001905, WO 2001/24729. Incontinence articles are described in thefollowing references: Disposable Absorbent Article for IncontinentIndividuals: EP 311344 description pages 3-9; Disposable AbsorbentArticle: EP 850623; Absorbent Article: WO 95/26207; Absorbent Article:EP 894502; Dry-Laid Fibrous Structure: EP 850 616; WO 98/22063; WO97/49365; EP 903134; EP 887060; EP 887059; EP 887058; EP 887057; EP887056; EP 931530; WO 99/25284; WO 98/48753. Feminine care andincontinence articles are described in the following references:Catamenial Device: WO 93/22998 description pages 26-33; AbsorbentMembers for Body Fluids: WO 95/26209 description pages 36-69; DisposableAbsorbent Article: WO 98/20916 description pages 13-24; ImprovedComposite Absorbent Structures: EP 306262 description pages 3-14; BodyWaste Absorbent Article: WO 99/45973. These references and thereferences therein are hereby expressly incorporated herein.

The hydrogel-forming polymers according to the present invention arevery useful as absorbents for water and aqueous fluids, so that they maybe used with advantage as a water retainer in market gardening, as afilter aid and particularly as an absorbent component in hygienearticles such as diapers, tampons or sanitary napkins.

Incorporation and fixation of the highly swellable hydrogels accordingto the present invention

In addition to the above-described highly swellable hydrogels, theabsorbent composition of the present invention includes constructionswhich include highly swellable hydrogels or to which they are fixed. Anyconstruction is suitable that is capable of accommodating highlyswellable hydrogels and of being integrated into the absorption layer. Amultiplicity of such compositions is already known and described indetail in the literature. A construction for installing the highlyswellable hydrogels can be for example a fiber matrix consisting of acellulose fiber mixture (air-laid web, wet laid web) or syntheticpolymer fibers (meltblown web, spunbonded web) or else of a fiber blendof cellulose fibers and synthetic fibers. Possible fiber materials aredetailed in the chapter which follows. The air-laid web process isdescribed for example in WO 98/28 478. Furthermore, open-celled foams orthe like may be used to install highly swellable hydrogels.

Alternatively, such a construction can be the result of fusing twoindividual layers to form one or better a multiplicity of chambers whichcontain the highly swellable hydrogels. Such a chamber system isdescribed in detail in EP 0 615 736 A1 page 7 lines 26 et seq.

In this case, at least one of the two layers should be water pervious.The second layer may either be water pervious or water impervious. Thelayer material used may be tissues or other fabric, closed oropen-celled foams, perforated films, elastomers or fabrics composed offiber material. When the absorbent composition consists of aconstruction of layers, the layer material should have a pore structurewhose pore dimensions are small enough to retain the highly swellablehydrogel particles. The above examples of the construction of theabsorbent composition also include laminates composed of at least twolayers between which the highly swellable hydrogels are installed andfixed.

Generally it is possible to fix hydrogel particles within the absorbentcore to improve dry and wet integrity. Dry and wet integrity describesthe ability to install highly swellable hydrogels into the absorbentcomposition in such a way that they withstand external forces not onlyin the wet but also in the dry state and highly swellable polymer doesnot dislocate or spill out. The forces referred to are especiallymechanical stresses as occur in the course of moving about while wearingthe hygiene article or else the weight pressure on the hygiene articlein the case of incontinence especially. As to fixation, one skilled inthe art knows a multiplicity of possibilities. Examples such as fixationby heat treatment, addition of adhesives, thermoplastics, bindermaterials are noted in WO 95/26 209 page 37 line 36 to page 41 line 14.The cited passage is thus part of this invention. Methods for enhancingwet strength are also to be found in WO 2000/36216 A1.

Furthermore, the absorbent composition may comprise a base material, forexample a polymer film on which the highly swellable hydrogel particlesare fixed. The fixing may be effected not only on one side but also onboth sides. The base material can be water pervious or water impervious.

The above constructions of the absorbent composition incorporate thehighly swellable hydrogels at a weight fraction of from 10-100% byweight, preferably 20-100% by weight, more preferably 30-100% by weight,even more preferably 40-100% by weight, much more preferably 50-100% byweight, particularly preferably 60-100% by weight, especially preferably70-100% by weight, extremely preferably 80-100% by weight and mostpreferably 90-100% by weight, based on the total weight of theconstruction and of the highly swellable hydrogels.

Fiber Materials of the Absorbent Composition

The structure of the present absorbent composition according to theinvention may be based on various fiber materials, which are used as afiber network or matrices. The present invention includes not onlyfibers of natural origin (modified or unmodified) but also syntheticfibers.

A detailed overview of examples of fibers which can be used in thepresent invention is given in WO 95/26 209 page 28 line 9 to page 36line 8. The cited passage is thus part of this invention.

Examples of cellulose fibers include cellulose fibers which arecustomarily used in absorption products, such as fluff pulp andcellulose of the cotton type. The materials (soft- or hardwoods),production processes such as chemical pulp, semichemical pulp,chemothermomechanical pulp (CTMP) and bleaching processes are notparticularly restricted. For instance, natural cellulose fibers such ascotton, flax, silk, wool, jute, ethylcellulose and cellulose acetate areused.

Suitable synthetic fibers are produced from polyvinyl chloride,polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene chloride,polyacrylic compounds such as ORLON®, polyvinyl acetate, polyethyl vinylacetate, soluble or insoluble polyvinyl alcohol. Examples of syntheticfibers include thermoplastic polyolefin fibers, such as polyethylenefibers (PULPEX®), polypropylene fibers and polyethylene-polypropylenebicomponent fibers, polyester fibers, such as polyethylene terephthalatefibers (DACRON® or KODEL®), copolyesters, polyvinyl acetate, polyethylvinyl acetate, polyvinyl chloride, polyvinylidene chloride,polyacrylics, polyamides, copolyamides, polystyrene and copolymers ofthe aforementioned polymers and also bicomponent fibers composed ofpolyethylene terephthalate-polyethylene-isophthalate copolymer,polyethyl vinyl acetate/polypropylene, polyethylene/polyester,polypropylene/polyester, copolyester/polyester, polyamide fibers(nylon), polyurethane fibers, polystyrene fibers and polyacrylonitrilefibers. Preference is given to polyolefin fibers, polyester fibers andtheir bicomponent fibers. Preference is further given to thermallyadhesive bicomponent fibers composed of polyolefin of the core-sheathtype and side-by-side type on account of their excellent dimensionalstability following fluid absorption.

The synthetic fibers mentioned are preferably used in combination withthermoplastic fibers. In the course of the heat treatment, the lattermigrate to some extent into the matrix of the fiber material present andso constitute bond sites and renewed stiffening elements on cooling.Additionally the addition of thermoplastic fibers means that there is anincrease in the present pore dimensions after the heat treatment hastaken place. This makes it possible, by continuous addition ofthermoplastic fibers during the formation of the absorbent layer, tocontinuously increase the fraction of thermoplastic fibers in thedirection of the topsheet, which results in a similarly continuousincrease in the pore sizes. Thermoplastic fibers can be formed from amultiplicity of thermoplastic polymers which have a melting point ofless than 190° C., preferably in the range from 75° C. to 175° C. Thesetemperatures are too low for damage to the cellulose fibers to belikely.

Lengths and diameters of the above-described synthetic fibers are notparticularly restricted, and generally any fiber from 1 to 200 mm inlength and from 0.1 to 100 denier (gram per 9000 meters) in diameter maypreferably be used. Preferred thermoplastic fibers are from 3 to 50 mmin length, particularly preferred thermoplastic fibers are from 6 to 12mm in length. The preferred diameter for the thermoplastic fiber is inthe range from 1.4 to 10 decitex, and the range from 1.7 to 3.3 decitex(gram per 10 000 meters) is particularly preferred. The form of thefiber may vary; examples include woven types, narrow cylindrical types,cut/chopped yarn types, staple fiber types and continuous filament fibertypes.

The fibers in the absorbent composition of the present invention can behydrophilic and/or hydrophobic. According to the definition of Robert F.Gould in the 1964 American Chemical Society publication “Contact angle,wettability and adhesion”, a fiber is referred to as hydrophilic whenthe contact angle between the liquid and the fiber (or the fibersurface) is less than 90° or when the liquid tends to spreadspontaneously on the same surface. The two processes are generallycoexistent. Conversely, a fiber is termed hydrophobic when a contactangle of greater than 90° is formed and no spreading is observed.

Preference is given to using hydrophilic fiber material. Particularpreference is given to using fiber material which is weakly hydrophilicon the body side and most hydrophilic in the region surrounding thehighly swellable hydrogels. In the manufacturing process, layers havingdifferent hydrophilicities are used to create a gradient which channelsimpinging fluid to the hydrogel, where it is ultimately absorbed.

Suitable hydrophilic fibers for use in the absorbent composition of thepresent invention include for example cellulose fibers, modifiedcellulose fibers, rayon, polyester fibers, for example polyethyleneterephthalate (DACRON®), and hydrophilic nylon (HYDROFIL®). Suitablehydrophilic fibers may also be obtained by hydrophilicizing hydrophobicfibers, for example the treatment of thermoplastic fibers obtained frompolyolefins (e.g. polyethylene or polypropylene, polyamides,polystyrenes, polyurethanes, etc.) with surfactants or silica. However,for cost reasons and ease of availability, cellulosic fibers arepreferred.

The highly swellable hydrogel particles are embedded into the fibermaterial described. This can be done in various ways, for example byusing the hydrogel material and the fibers together to create anabsorbent layer in the form of a matrix, or by incorporating highlyswellable hydrogels into fiber mixture layers, where they are ultimatelyfixed, whether by means of adhesive or lamination of the layers.

The fluid-acquiring and -distributing fiber matrix may comprisesynthetic fiber or cellulosic fiber or a mixture of synthetic fiber andcellulosic fiber, in which case the mixing ratio may vary from (100 to0) synthetic fiber: (0 to 100) cellulosic fiber. The cellulosic fibersused may additionally have been chemically stiffened to increase thedimensional stability of the hygiene article.

The chemical stiffening of cellulosic fibers may be provided indifferent ways. A first way of providing fiber stiffening is by addingsuitable coatings to the fiber material. Such additives include forexample polyamide-epichlorohydrin coatings (Kymene® 557H, Hercoles, Inc.Wilmington, Del., USA), polyacrylamide coatings (described in U.S. Pat.No. 3,556,932 or as the Parez® 631 NC commercial product from AmericanCyanamid Co., Stamford, Conn., USA), melamine-formaldehyde coatings andpolyethyleneimine coatings.

Cellulosic fibers may also be chemically stiffened by chemical reaction.For instance, suitable crosslinker substances may be added to effectcrosslinking taking place within the fiber. Suitable crosslinkersubstances are typical substances used for crosslinking monomersincluding but not limited to C₂-C₈-dialdehydes, C₂-C₈-monoaldehydeshaving acid functionality and in particular C₂-C₉-polycarboxylic acids.Specific substances from this series are for example glutaraldehyde,glyoxal, glyoxylic acid, formaldehyde and citric acid. These substancesreact with at least 2 hydroxyl groups within any one cellulose chain orbetween two adjacent cellulose chains within any one cellulose fiber.The crosslinking causes a stiffening of the fibers, to which greaterdimensional stability is imparted as a result of this treatment. Inaddition to their hydrophilic character, these fibers exhibit uniformcombinations of stiffening and elasticity. This physical property makesit possible to retain the capillary structure even under simultaneouscontact with fluid and compressive forces and to prevent prematurecollapse.

Chemically crosslinked cellulose fibers are known and described in WO91/11162, U.S. Pat. No. 3,224,926, U.S. Pat. No. 3,440,135, U.S. Pat.No. 3,932,209, U.S. Pat. No. 4,035,147, U.S. Pat. No. 4,822,453, U.S.Pat. No. 4,888,093, U.S. Pat. No. 4,898,642 and U.S. Pat. No. 5,137,537.The chemical crosslinking imparts stiffening to the fiber material,which is ultimately reflected in improved dimensional stability for thehygiene article as a whole. The individual layers are joined together bymethods known to one skilled in the art, for example intermelting byheat treatment, addition of hot-melt adhesives, latex binders, etc.

Methods of Making the Absorbent Composition

The absorbent composition is composed of constructions which containhighly swellable hydrogels and the highly swellable hydrogels which arepresent in said constructions or fixed thereto.

Examples of processes to obtain an absorbent composition comprising forexample a base material to which highly swellable hydrogels are fixed onone or both sides are known and included by the invention but notlimited thereto.

Examples of processes to obtain an absorbent composition comprising forexample a fiber material blend of synthetic fibers (a) and cellulosefibers (b) embedded in highly swellable hydrogels (c), the blend ratiovarying from (100 to 0) synthetic fiber: (0 to 100) cellulose fiber,include (1) a process where (a), (b) and (c) are mixed together at oneand the same time, (2) a process where a mixture of (a) and (b) is mixedinto (c), (3) a process where a mixture of (b) and (c) is mixed with(a), (4) a process where a mixture of (a) and (c) is mixed into (b), (5)a process where (b) and (c) are mixed and (a) is continuously meteredin, (6) a process where (a) and (c) are mixed and (b) is continuouslymetered in, and (7) a process where (b) and (c) are mixed separatelyinto (a). Of these examples, processes (1) and (5) are preferred. Theapparatus used in this process is not particularly restricted and anycustomary apparatus known to one skilled in the art can be used.

The absorbent composition obtained in this way can optionally besubjected to a heat treatment, so that an absorption layer havingexcellent dimensional stability in the moist state is obtained. The heattreatment process is not particularly restricted. Examples include heattreatment by feeding hot air or infrared irradiation. The temperature ofthe heat treatment is in the range from 60° C. to 230° C., preferablyfrom 100° C. to 200° C., particularly preferably from 100° C. to 180° C.

The duration of the heat treatment depends on the type of syntheticfiber, its amount and the hygiene article production rate. Generally theduration of the heat treatment is in the range from 0.5 second to 3minutes, preferably from 1 second to 1 minute.

The absorbent composition is generally provided for example with aliquid-pervious topsheet and a liquid-impervious backsheet. Furthermore,leg cuffs and adhesive tabs are attached to finalize the hygienearticle. The materials and types of pervious topsheet and imperviousbacksheet and of the leg cuffs and adhesive tabs are known to oneskilled in the art and are not particularly restricted. Examples thereofmay be found in WO 95/26 209.

The present invention is advantageous in that the esters F, which areuseful as crosslinkers, do not have to be purified after they have beenformed and particularly in that the (meth)acrylic acid, preferablyacrylic acid, does not have to be removed, since it is generally amonomer for forming the hydrogels.

Experimental Part

Parts per million and percentages are by weight, unless otherwisestated.

The example which follows illustrates the process of the presentinvention.

EXAMPLES Production of Crude Acrylate Esters Useful as SAP-Crosslinkers

SAP-crosslinkers are prepared in the examples by esterifying alkoxylatedtrimethylolpropane with acrylic acid by removing water in an azeotropicdistillation. The esterification catalyst in the examples is sulfuricacid. The reactants are introduced in the examples as initial charge inmethylcyclohexane entrainer together with a stabilizer mixtureconsisting of hydroquinone monomethyl ether, triphenyl phosphite andhypophosphorous acid. The reaction mixture is then heated to about 98°C. until the azeotropic distillation starts. During the azeotropicdistillation, the temperature in the reaction mixture rises. The amountof water removed is determined. The distillation is discontinued once atleast the theoretical amount of water has been removed. Subsequently theentrainer is removed in a vacuum distillation. The product is cooled andused as a crosslinker in SAP production.

Conversion and yield of the reaction is not precisely determined becausethe water removed in the esterification also contains acrylic acid andacrylic acid is also removed during the vacuum distillation of theentrainer. Similarly, the crude ester still contains free acrylic acidwhich is titrated together with the catalyst (acid number).

Parts are by weight, unless otherwise stated.

Production of Ester

Acid numbers were determined in accordance with DIN EN 3682.

Example 1 Preparation of Alkoxylated Trimethylolpropane

77 g of trimethylolpropane are placed with 0.5 g of KOH 45% in water asan initial charge in an autoclave and dewatered at 80° C. and reducedpressure (about 20 mbar). 167 g of propylene oxide are then added at 120to 130° C. and allowed to react at this temperature under elevatedpressure. The reaction has ended when no further change in pressure isobserved. The reaction mixture is then stirred for a further 30 min atabout 120° C. 379 g of ethylene oxide is subsequently added at 145 to155° C. at elevated pressure over a prolonged period and likewiseallowed to react. After purging with inert gas and cooling down to 60°C., the catalyst is separated off by addition of sodium pyrophosphateand subsequent filtration.

Example 2 Preparation of Acrylic Ester

887 parts of approximately 5-tuply propoxylated and 15-tuply ethoxylatedtrimethylolpropane (as per example 1) is esterified with 216 parts ofacrylic acid and 5 parts of sulfuric acid in 345 parts ofmethylcyclohexane. The assistants used were 3 parts of hydroquinonemonomethyl ether, 1 part of triphenyl phosphite and 1 part ofhypophosphorous acid. 44 parts of water were removed before theentrainer was removed by vacuum distillation. The product was purifiedthrough K300 filter. The acid number is determined. The viscosity isadjusted by addition of 96 parts of acrylic acid. The viscosity of thealmost colorless product (iodine color number 0-1) is about 320 mPas.

Making of Hydrogels

To determine the quality of surface crosslinking, the dried hydrogel canbe investigated using the following test methods.

Test Methods

a) Centrifuge Retention Capacity (CRC)

This method measures the free swellability of the hydrogel in a teabag.0.2000*0.0050 g of dried hydrogel (particle size fraction 106-850 μm)are weighed into a teabag 60×85 mm in size which is subsequently sealed.The teabag is placed for 30 minutes in an excess of 0.9% by weightsodium chloride solution (at least 0.83 l of sodium chloride solution/1g of polymer powder). The teabag is then centrifuged for 3 minutes at250 g. The amount of liquid is determined by weighing back thecentrifuged teabag.

b) Absorbency Under Load (AUL) (0.7 psi)

The measuring cell for determining AUL 0.7 psi is a Plexiglass cylinder60 mm in internal diameter and 50 mm in height. Adhesively attached toits underside is a stainless steel sieve bottom having a mesh size of 36μm. The measuring cell further includes a plastic plate having adiameter of 59 mm and a weight which can be placed in the measuring celltogether with the plastic plate. The plastic plate and the weighttogether weigh 1345 g. AUL 0.7 psi is determined by determining theweight of the empty Plexiglass cylinder and of the plastic plate andrecording it as W₀. 0.900*0.005 g of hydrogel-forming polymer (particlesize distribution 150-800 μm) is then weighed into the Plexiglasscylinder and distributed very uniformly over the stainless steel sievebottom. The plastic plate is then carefully placed in the Plexiglasscylinder, the entire unit is weighed and the weight is recorded asW_(a). The weight is then placed on the plastic plate in the Plexiglasscylinder. A ceramic filter plate 120 mm in diameter and 0 in porosity isthen placed in the middle of a Petri dish 200 mm in diameter and 30 mmin height and sufficient 0.9% by weight sodium chloride solution hisintroduced for the surface of the liquid to be level with the filterplate surface without the surface of the filter plate being wetted. Around filter paper 90 mm in diameter and <20 μm in pore size (S&S 589Schwarzband from Schleicher & Schüll) is subsequently placed on theceramic plate. The Plexiglass cylinder containing hydrogel-formingpolymer is then placed with plastic plate and weight on top of thefilter paper and left there for 60 minutes. At the end of this period,the complete unit is removed from the filter paper and the Petri dishand subsequently the weight is removed from the Plexiglass cylinder. ThePlexiglass cylinder containing swollen hydrogel is weighed together withthe plastic plate and the weight recorded as W_(b).

AUL was calculated by the following equation:AUL 0.7psi [g/g]=[W _(b) −W _(a) ]/[W _(a) −W ₀]

AUL 0.5 psi is measured in similar fashion at a lower pressure.

c) The 16 h Extractables Value is Determined Similarly to theDescription in EP-A1 811 636 at Page 13 Line 1 to Line 19.

d) Method for Determining Residual Levels of Crosslinkers in Hydrogels

To determine the level of residual, unconverted crosslinker, thisresidual crosslinker is initially extracted from the dried hydrogel by adouble extraction. To this end, 0.400 g of dry hydrogel and 40 g of 0.9%by weight sodium chloride solution are weighed into a sealable andcentrifugable ampoule. 8 ml of dichloromethane are added, the ampoule issealed and is then shaken for 60 min. The ampoule is thereafterimmediately centrifuged at 1500 rpm for 5 min, so that the organic phaseis cleanly separated from the aqueous phase.

50 μl of monoethylene glycol are weighed into a second ampoule, about5-6 ml of the dichloromethane extract are added, the weight of theextract is measured accurately to 0.001 g. The dichloromethane is thenevaporated off at 50-55° C. and the residue after cooling is taken upwith 2 ml of methanol-water mixture (50 parts by volume of each). Thisis followed by shaking for 10 min before filtration through a PTFE 0.45μm filter.

The sample thus obtained is separated by means of liquid phasechromatography and analyzed by mass spectrometry. Quantification isagainst a dilution series of the same crosslinker used.

The chromatography coiumn used is a Zorbax Eclipse XDB C-8 (150×4.6 mm-5μm) and the precolumn used is a Zorbax Eclipse XDB C-8 (12.5×4.6 mm-5μm). The mobile phase used is a 75/25 methanol/water mixture.

The gradient course is as follows:

Time (min) % Methanol % Water 0 75 25 3 75 25 4 98 2 8 98 2 9 75 25 1475 25

-   Flow is 1 ml/min at 1600 psi pressure.-   The injection volume is 20 μl.-   Typical analysis time is 14 min for the samples.

Detection is by mass spectrometry, for example in the range 800-1300 m/z(full scan, positive). The instrument utilizes APCI (atmosphericpressure chemical ionization, positive ionization). For optimization,the capillary temperature is set to 180° C., the APCI vaporizertemperature to 450° C., source current to 5.0 μA and gas flow to 80ml/min.

The individual settings have to be done separately for each crosslinker.To this end, a suitable calibrating solution of the crosslinker is usedto determine the characteristic peaks which are later relevant forevaluation. The main peak is generally chosen.

The residual crosslinker concentration is then calculated as follows:CONC _(Probe) =A _(Probe) ×CONC _(Std) ×VF/A _(Std)

-   CONC_(Probe): is wanted residual crosslinker concentration in dry    hydrogel in mg/kg-   CONC_(Std): is wanted residual crosslinker concentration in    calibrating solution in mg/kg-   A_(Probe): is peak area of extract sample of dried hydrogel-   A_(Std): is peak area of calibrating solution-   VF is the dilution factor:    VF=M _(DCM) ×M _(Solv)/(M _(probe) ×M _(Extract))-   M_(DCM) is weight of dichloromethane for extraction-   M_(Probe) is weight of dry hydrogel-   M_(Solv) is weight of methanol-water mixture+monoethylene glycol-   M_(Extract) is weight of dichloromethane extract

A calibration has to be carried out (involving a plurality of points inthe range 0-50 ppm for example) to ensure that the determination iscarried out in the linear range.

e) Saponification Index VSI

The comminuted gel is then further treated in two different ways:

Workup Method 1:

The comminuted gel is evenly spread out in a thin layer onsieve-bottomed trays and then dried at 80° C. under reduced pressure for24 h. This form of drying is very gentle on the product and thereforerepresents the best standard for comparison.

The dried hydrogel is then ground and the sieve fraction of 300-600micrometers is isolated.

Workup Method 2:

The comminuted gel is initially heat-treated at 90° C. in a sealedplastic bag for 24 h. It is then spread out evenly in a thin layer onsieve-bottomed trays and dried at 80° C. under reduced pressure for 24h. This drying simulates the drying conditions which occur in typicalmanufacturing plants and which customarily limit the drying performanceand the throughput because of the reduced quality associated therewith.

The dried hydrogel is ground and the sieve fraction of 300-600micrometers is isolated.

The hydrogels obtained according to the two workup methods arecharacterized by determination of teabag capacity (CRC) and also of theextractables content after 16 h and with regard to the level ofunreacted, residual crosslinker. In addition, the moisture content isdetermined and if found to be above 1% by weight it is arithmeticallyallowed for when determining these properties. Typically, the moisturecontent will be about 5% by weight.

The measured values are then used to determine the saponification index(VSI) of the crosslinker in the gel, which computes as follows:VSI=0.5×(CRC ₂ −CRC ₁)+0.5×(extractables₂-extractables₁)

The subscripted indices here indicate workup method 1 and workup method2, as the case may be. Thus, the saponification index increases whenteabag capacity increases as a result of plant drying and when thefraction of extractables increases in the process. The two contributionsare given equal weight.

It is generally advantageous to use crosslinkers whose saponificationindex is very small. The ideal crosslinker has a VSI of zero. The use ofsuch crosslinkers makes it possible to increase the performance of theplant dryers to the technically achievable maximum without loss ofquality. The reason for this is that the degree of crosslinking achievedduring the polymerization—and hence the properties of the endproduct—does not change any more by hydrolysis in the course of drying.

Example 3 Preparation of Superabsorbent Using the Acrylic Ester ofExample 2 and Other Internal Crosslinkers Example a

305 g of acrylic acid and 3204 g of a 37.3% by weight sodium acrylatesolution are dissolved in 1465 g of distilled water in an acid-resistantplastics tub. 12.2 g of TMP 15EO triacrylate are added as a crosslinkerand also 0.61 g of V-50 (2,2′-azobis-amidinopropane dihydrochloride) and3.05 g of sodium persulfate as initiators. The initiators areadvantageously predissolved in a portion of the batch water. The batchis thoroughly stirred for some minutes.

Then nitrogen gas is bubbled through the plastics film covered solutionin the tub for about 30 min in order that oxygen may be removed and ahomogeneous distribution may be achieved for the crosslinker. Finally,0.244 g of hydrogen peroxide dissolved in 5 g of water and also 0.244 gof ascorbic acid dissolved in 5 g of water are added. The temperature atthe start of the reaction should be 11-13° C. The reaction solution isabout 6 cm deep. The reaction starts after a few minutes and is allowedto proceed under adiabatic conditions and the thermally insulated tub isallowed to stand thermally for not longer than 30 min before the gel isworked up.

To work up the gel, the gel block is initially broken into pieces andthen comminuted through a meat grinder equipped with a 6 mm breakerplate.

The comminuted gel is then further treated in two different ways:

Workup Method 1

The comminuted gel is evenly spread out in a thin layer onsieve-bottomed trays and then dried at 80° C. under reduced pressure for24 h. This form of drying is very gentle on the product and thereforerepresents the best standard for comparison.

The dried hydrogel is then ground and the sieve fraction of 300-600micrometers is isolated.

Workup Method 2:

The comminuted gel is initially heat-treated at 90° C. in a sealedplastic bag for 24 h. It is then spread out evenly in a thin layer onsieve-bottomed trays and dried at 80° C. under reduced pressure for 24h. This drying simulates the drying conditions which occur in typicalmanufacturing plants and which customarily limit the drying performanceand the throughput because of the reduced quality associated therewith.

The dried hydrogel is ground and the sieve fraction of 300-600micrometers is isolated.

The following further examples are prepared similarly to example a:

TABLE 1 Amount Example Amount used based on used No. Crosslinker typeacrylic acid monomer in g a TMP - 3 EO triacrylate 1% by weight 12.2 g bTMP - 15 EO triacrylate 1% by weight 12.2 g c TMP - 20 EO triacrylate 1%by weight 12.2 g d TMP - 5 PO - 15 EO 1% by weight 12.2 g triacrylate

The properties achieved for these hydrogels are summarized in tab. 2:

Crosslinker Crosslinker CRC 1 Extractables residue 1 CRC 2 Extractablesresidue 2 Ex. [g/g] 16 h 1[wt %] [ppm] [g/g] 16 h 2 [wt %] [ppm] VSI aTMP-3 EO 36.6 4.4 857 70.6 44.2 1302 36.9 b TMP-15 EO 29.7 2.8 51 43.112.6 20 11.6 c TMP-20 EO 30.3 2.9 29 41.1 13.1 14 10.5 d TMP-5 PO-15 EO29.7 2.7 18 38.7 11.0 <10 8.7

Example 4a Preparation of a Superabsorbent Using the Acrylic Ester ofExample 2

A Lödige VT 5R-MK plowshare kneader (5 l volume) is charged with 388 gof deionized water, 173.5 g of acrylic acid, 2033.2 g of a 37.3% byweight sodium acrylate solution (100 mol % neutralized) and also 5.90 gof the crosslinker trimethylolpropane-5 PO-15 EO triacrylate prepared inexample 2. This initial charge is inertized by having nitrogen bubbledthrough it for 20 minutes. Dilute aqueous solutions of 2.112 g of sodiumpersulfate, 0.045 g of ascorbic acid and also 0.126 g of hydrogenperoxide are then added to start the reaction at about 23° C. After thereaction has started, the temperature of the heating jacket iscontrolled to the reaction temperature in the reactor. The crumbly geleventually obtained is then dried in a circulating air drying cabinet at160° C. for about 3 h. This is followed by grinding and classifying to300-850 micrometers. The hydrogel obtained is then surfacepostcrosslinked.

Example 4b

Similar to example 4a, except that the amount of crosslinker used israised to 12 g.

Example 5a Comparative Example

Very similar to example 4a, except that the crosslinkertrimethylolpropane-15 EO-5 PO triacrylate is used. The gel obtained isclumpy and has to be comminuted in a meat grinder before drying.

Example 5b Comparative Example

Similar to example 5a, except that the amount of the crosslinker used israised to 12 g.

Postcrosslinking:

The dry base polymer powder from examples 4 and 5 is sprayedhomogeneously (while stirring) with a solution of 0.10% by weight ofethylene glycol diglycidyl ether (from Nagase, Japan), 3.43% by weightof water and 1.47% by weight of 1,2-propanediol, each percentage beingbased on polymer used.

The moist powder is then heat treated in a drying cabinet at 150° C. for60 min. It is then sieved once more at 850 micrometers in order thatagglomerates may be removed. The properties of this postcrosslinkedpolymer are determined.

The properties of the postcrosslinked polymers of examples 4 and 5 andalso of further variants are summarized in tab. 3:

Ex- Amount ample Crosslinker type used CRC AAP 0.3 psi AAP 0.7 psi 4aTMP-5PO-15EO 5.9 g 36 26 16 triacrylate 5a TMP-15EO-5PO 5.9 g 41 17 10triacrylate 4b TMP-5PO-15EO 12 g 31 33 26 triacrylate 5b TMP-15EO-5PO 12g 37 23 13 triacrylate

Only the crosslinker used in examples 4a and 4b evidently leads toproduct properties typical of state of the art superabsorbents.

The crosslinker used in 5a and 5b, what is more, only leads to verytough and difficult-to-process gels, which are difficult to prepare in akneader.

1. An ester F of formula I

wherein EO is O—CH2-CH2-, PO is independently at each instanceO—CH2-CH(CH3)- or O—CH(CH3)-CH2-, n1, n2, and n3 are independently 4, 5,or 6, n1+n2+n3 is 14, 15, or 16, m1, m2, and m3 are independently 1,2,or 3, m1+m2+m3 is 4, 5, or 6, and R1, R2, and R3 are independently H orCH3.
 2. The ester F of claim 1 wherein n1+n2+n3 is
 15. 3. The ester F ofclaim 1 wherein n1=n2=n3=5.
 4. The ester F of claim 1 wherein m1+m2+m3is
 5. 5. The ester F of claim 1 wherein m1=m2=2 and m3=1.
 6. The ester Fof claim 1 wherein R1, R2, and R3 are identical.
 7. A process forpreparing an ester F of claim 1 from an alkoxylated trimethylolpropaneof formula II

wherein EO, PO, n1, n2, n3, m1, m2, and m3 are each as defined in claim1, and (meth)acrylic acid, comprising the steps of a) reacting thealkoxylated trimethyloipropane II with (meth)acrylic acid in thepresence of at least one esterification catalyst C, at least onepolymerization inhibitor D, and optionally a water-azeotroping solvent Eto form the ester F, b) optionally removing from the reaction mixturesome or all of the water formed in a), during and/or after a), f)optionally neutralizing the reaction mixture, h) when a solvent E isused, optionally removing the solvent E by distillation, and/or i)stripping the reaction mixture with a gas which is inert under thereaction conditions, wherein a molar excess of(meth)acrylic acid toalkoxylated trimethylolpropane in step (a) is at least 3.15:1.
 8. Theprocess of claim 7 wherein the optionally neutralized (meth)acrylic acidpresent in the reaction mixture after the last process stepsubstantially remains in the reaction mixture.
 9. The process of claim 7wherein the (meth)acrylic acid is not more than 75% by weight removedfrom the reaction mixture obtained after the last step, which reactionmixture contains the ester F.
 10. The process of claim 7 wherein thereaction mixture obtained after the last process step, which containsthe ester F, has a DIN EN 3682 acid number of at least 25 mg of KOH/g.11. The process of claim 7 wherein the reaction mixture obtained afterthe last process step, which contains the ester F, has a (meth)acrylicacid content of at least 0.5% by weight.
 12. The process of claim 7wherein the molar ratio of (meth)acrylic acid to alkoxylatedtrimethyloipropane in step a) is at least 15:1.
 13. A process forpreparing a crosslinked hydrogel comprising the steps of k) polymerizingan ester F of claim 1 with (meth)acrylic acid, optionally with anadditional monoethylenically unsaturated compound N, and optionally atleast one further copolymerizable hydrophilic monomer M, in the presenceof at least one free-radical initiator K and optionally at least onegrafting base L, l) optionally postcrosslinking the reaction mixtureobtained from k), m) drying the reaction mixture obtained from k) or l),and n) optionally grinding and/or sieving the reaction mixture obtainedfrom k), l), or m).
 14. A process for preparing a crosslinked hydrogelcomprising steps a) to i) of claim 7 and additionally k) polymerizingthe reaction mixture from one of steps a) to i) of claim 7, ifperformed, optionally with an additional monoethylenically unsaturatedcompound N and optionally at least one further copolymerizablehydrophilic monomer M, in the presence of at least one free-radicalinitiator K and optionally at least one grafting base L, l) optionallypostcrosslinking the reaction mixture obtained from k), m) drying thereaction mixture obtained from k) or l), and n) optionally grindingand/or sieving the reaction mixture obtained from k), l), or m).
 15. Apolymer prepared according to the process of claim
 13. 16. A crosslinkedhydrogel containing at least one hydrophilic monomer M in polymerizedform crosslinked with an ester F of claim
 1. 17. A compositioncomprising from 0.1% to 40% by weight of at least one ester F of claim1, 0.5-99.9% by weight of at least one hydrophilic monomer M, 0-10% byweight of at least one esterification catalyst C, 0-5% by weight of atleast one polymerization inhibitor D, and 0-10% by weight of a solventE, with the proviso that the sum total is always 100% by weight.
 18. Thecomposition of claim 17 further comprising a diluent G.
 19. Acrosslinked hydrogel prepared from a composition of claim 17, andoptionally postcrosslinked.
 20. A crosslinked hydrogel having asaponification index of less than
 10. 21. A crosslinked hydrogelprepared according to claim 13 having a saponification index of lessthan
 10. 22. The ester F of claim 1 wherein R1, R2, and R3 are H.
 23. Apolymer prepared according to the process of claim
 14. 24. An articlecomprising a polymer prepared according to the method of claim
 13. 25.The article of claim 24 selected from the group consisting of a hygienearticle, a packaging material, and a nonwoven.
 26. The crosslinkedhydrogel of claim 20 having a saponification index of less than
 8. 27.The crosslinked hydrogel of claim 21 having a saponification index ofless than 9.